Microphytoplankton from mid dle palaeolatitudes of the South ern Hemi sphere – a re cord from cli mate change strata of Baltica’s O/S bound ary
Monika MASIAK1, Marzenna STEMPIEÑ-SA£EK2 and Teresa PODHALAÑSKA3, *
1 momas@gazeta.pl
2 Uni ver sity of Gdañsk, Fac ulty of Ocean og ra phy and Ge og ra phy, Marsza³ka Pi³sudskiego 46, 81-378 Gdynia, Po land
3 Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Rakowiecka 4, 00-975 Warszawa, Po land
Masiak, M., Stempieñ-Sa³ek, M., Podhalañska, T., 2020. Microphytoplankton from mid dle palaeolatitudes of the South ern Hemi sphere – a re cord from cli mate change strata of Baltica’s O/S bound ary. Geo log i cal Quar terly, 64 (1): 86–103, doi:
10.7306/gq.1518
In the pres ent study, data on the di ver sity of acritarch and prasinophyte microphytoplankton were col lected from Or do vi - cian–Si lu rian tran si tional strata at three dif fer ent geo log i cal lo cal i ties in Po land: south ern part of the Holy Cross Moun tains (Ma³opolska Block, south ern Po land), East Eu ro pean Plat form (Baltica), and Koszalin–Chojnice Zone (NW Po land). The ma - te rial rep re sents the mucronata (tri lo bite) to vesiculosus (graptolite) biozones. The Or do vi cian microphytoplankton as sem - blages are char ac ter ized by low fre quency and low di ver sity: up to 110 spec i mens and 12 gen era per slide, in con trast to Si lu rian ones that are more fre quent and more di verse – >3000 spec i mens and 16 gen era per slide (di ver sity at the spe cies level is also higher). Through out the Or do vi cian part of the suc ces sion, typ i cal Or do vi cian gen era oc cur, such as Acanthodiacrodium, Ordovicidium, Orthosphaeridium and large Baltisphaeridium, to gether with iso lated oc cur rences of typ - i cal Si lu rian gen era, es pe cially in the up per part of the Hirnantian (e.g., Diexallophasis). The Si lu rian as sem blages are typ i - fied by high fre quency of prasinophytes fol lowed by typ i cal Si lu rian acritarchs: Tylotopalla, Ammonidium, Domasia and Oppilatala. The palynological ma te rial from Po land is com pared with ma te rial known from other lo cal i ties: the Rapla and Valga bore holes (Es to nia) and Anticosti Is land (Can ada), as well as oth ers placed in dif fer ent palaeocontinents and dif fer ent bathymetric zones. Palynological as sem blages ob tained from Pol ish ma te rial con firm that bathymetry played the key role in tax o nomic di ver si fi ca tion of the microphytoplankton as sem blage. This sup ports the ex ist ing mod els of dis tri bu tion: dom i - nance of prasinophytes (leiospheres) and cryptospores is char ac ter is tic for shal low-wa ter en vi ron ments. In deeper wa ter, dom i nance of acanthomorphs is ob served. In the deep est-wa ter zones – mixed as sem blages oc cur. For the Holy Cross Moun tains, microphytoplankton fre quency is com pared with a TOC curve in the same in ter val.
Key words: Po land, Or do vi cian/Si lu rian bound ary, palynology, acritarch fre quency, bathymetry, cli mate.
INTRODUCTION
One of the lon gest, most sig nif i cant and sus tained in ter vals of bi otic di ver si fi ca tion in the his tory of ma rine life on Earth oc - curred in the Or do vi cian Pe riod (Miller and Mao, 1995; Miller, 1997; Servais et al., 2008). Evo lu tion ary di ver si fi ca tion dur ing the Early and Mid dle Or do vi cian was in ter rupted in the lat est Or do vi - cian (Hirnantian) by one of most in tense mass ex tinc tion events of the Pa leo zoic Era. The end-Or do vi cian mass ex tinc tion is as - so ci ated with cli mate change and re gres sion ac com pa nied by a ma jor gla ci ation on the Gond wana supercontinent (Beuf et al., 1971; Brenchley, 1988; Finney et al., 1997). Raup and Sepkoski (1982) cal cu lated that ~22% of all ma rine fam i lies, 40% of ma - rine gen era (Sepkoski, 1996) and 85% of ma rine spe cies (Jablonski, 1991) be came ex tinct in the Late Or do vi cian.
Dur ing the Early Si lu rian, the global eco sys tem grad u ally re - cov ered from this mass ex tinc tion – dur ing it, no ma jor tax o nomic groups dis ap peared – and biodiversity rose again dur ing warmer palaeoclimatic con di tions (Armstrong, 1996, 2007; Berry et al., 1996; Armstrong and Harper, 2014). The char ac ter is tics of microphytoplankton re sponse to the Hirnantian bi otic cri sis and fol low ing the early Si lu rian re cov ery are still in com pletely con - strained. A gen eral re duc tion in both di ver sity and fre quency of acritarchs at the gen era and spe cies level dur ing the Late Or do - vi cian has been re corded in var i ous lo cal i ties world wide (Paris et al., 2000; Servais et al., 2008; Stempieñ-Sa³ek, 2011). The Early Si lu rian (Llandovery) is char ac ter ized by a re sur gence of amount and turn over: ap pear ance of new taxa and ex pan sion of taxa that ap peared right at the end of the Or do vi cian (those of Si lu rian “af - fin ity” sensu Le Hérissé in Paris et al., 2007:103).
The global and re gional pat terns of acritarch re cov ery af ter the Hirnantian mass ex tinc tion are rel a tively un known, mainly due to the scar city of de tailed stud ies. A ma jor prob lem af fect - ing the anal y sis of acritarch dis tri bu tion within stra tal suc ces - sions that span the Or do vi cian–Si lu rian bound ary is the lithofacies con trol on the pres er va tion of or ganic-walled microfossils (for ex am ple in Saudi Ara bia – Le Hérissé, 2000;
and Bo he mia – Dufka and Fatka, 1993).
* Cor re spond ing au thor, e-mail: teresa.podhalanska@pgi.gov.pl Re ceived: October 25, 2019; ac cepted: December 13, 2019; first pub lished on line: March 4, 2020
In the pres ent study, acritarch di ver sity data were col lected from Or do vi cian–Si lu rian tran si tional strata at three dif fer ent main lo cal i ties in Po land: Holy Cross Mts. (Ma³opolska Block), East Eu ro pean Plat form: Pomerania (Baltica), and West ern Pomerania (Koszalin–Chojnice Zone). All lo cal i ties are shown in Fig ure 1.
The most com plete re cord of microphytoplankton from O/S bound ary strata co mes from the Holy Cross Mts. (Ma³opolska Block) and poses a bench mark for as sem blages from other Pol ish suc ces sions. In the the Bardo Stawy suc ces sion, strata dem on strate the re sponse of acritarchs to palaeoenviron mental change through the lat est Or do vi cian and ear li est Si lu rian. The far-field ef fect of gla ci ation in Gond wana is ev i dent in the lat est Or do vi cian (Hirnantian), equated with a global cli mate cool ing that may have been a ma jor causal ef fect of mass ex tinc tion, in - clud ing plank tonic or gan isms such as graptolites, and im pact - ing on re duced di ver sity within acritarch as sem blages: this in - ter val is also as so ci ated with the wide spread ap pear ance of the cool/ven ti lated-wa ter Hirnantia fauna at lower lat i tudes (Rong et al., 2002). The global warm ing in the lat est Or do vi cian and ear li - est Si lu rian is as so ci ated likely with ocean o graphic change (tem per a ture, thermohaline cir cu la tion, etc.) that fa cil i tated the re cov ery as an in crease in fre quency (nu mer i cal abun dance
and biodiversity) of acritarchs. Lithofacies stud ies con firm the re gres sive na ture of Up per Or do vi cian de pos its re lated to the grow ing ice sheet of the South ern Hemi sphere. The max i mum cli mate cool ing and sea level re gres sion took place dur ing the early Hirnantian – extraordinarius graptolite Biozone us ing the Brit ish graptolite biozonation (Armstrong, 2007; Page et al., 2007). An equiv a lent of this zone in Po land is the Mucrionaspis mucronata tri lo bite Biozone due to the lack of graptolites (Kielan, 1956, 1959). In the up per most Hirnantian (persculptus graptolite Biozone), rep re sented by the deglaciation in ter val, the first signs of warm ing and eustatic change oc curred with the de po si tion of beige claystones and shales con tain ing silt- to sand-sized quartz grains, pre ced ing the fa cies change to dark
“grap to lit ic shales”. Such a lithological suc ces sion in di cates grad ual stag na tion and pro gres sive ox y gen de fi ciency of the depositional en vi ron ment re lated to a deep en ing of the sea dur - ing the post-gla cial sea level rise (Masiak et al., 2003). The re - sults pre sented here are a fur ther con tri bu tion to the un der - stand ing of the tim ing and causal mech a nisms of early Si lu rian acritarch re cov ery af ter the Hirnantian mass ex tinc tion and its pos si ble con nec tion with the as so ci ated palaeoclimate and palaeoenvironmental change.
Fig. 1A – ar eas of in ves ti ga tion (a map af ter geoportal.pgi.gov.pl, sup ple mented): East Eu ro pean Plat form, Bal tic De pres sion: 1 – Bia³ogóra 1 bore hole, 2 – £eba 8 bore hole, 3 – Koœcierzyna IG 1 bore hole and Koszalin–Chojnice Zone; 4 – Toruñ 1 bore hole; B – Bardo Stawy and Zalesie Nowe sec tions in the Holy Cross Moun tains, Kielce Re gion; palaeo ge ogra phy: C – Late Or do vi cian (af ter Torsvik and Cocks, 2005, sup ple mented), D – Early Si lu rian (af ter Nawrocki et al., 2007, sup ple mented)
GEOLOGICAL SETTING
The palynological ma te rial ana lysed in this pa per co mes from both out crops in cen tral Po land and bore holes in north ern Po land (Fig. 1A, B).
The study sec tions are lo cated in three dis tinct palaeo - geographic re gions (sensu Torsvik and Cocks, 2005; Nawrocki and Poprawa, 2006; Nawrocki et al., 2007) – the Ma³opolska Block (Kielce Re gion), Baltica (Bal tic de pres sion) and Koszalin–Chojnice Zone (Fig. 1A, B). All lo cal i ties, the Ma³opolska Block, Baltica and Koszalin–Chojnice Zone, oc cu - pied dif fer ent palaeogeographic po si tions dur ing Late Or do vi - cian Hirnantian time, al though the palaeolatitudes (~30°S) were sim i lar (Lewandowski, 1993; Bednarczyk, 1999; Cocks, 2000;
Cocks and Torsvik, 2005; Torsvik and Cocks, 2005). They were lo cated in the same ba sin around west ern Baltica. The palaeogeographic po si tions are shown in Fig ure 1C, D.
MA£OPOLSKA BLOCK
In the Early Or do vi cian the Ma³opolska Block was placed at
~60°S lat i tude (Lewandowski, 1993) and through the Or do vi - cian it moved north ward reach ing ~30°S lat i tude in the Late Or - do vi cian (Cocks and Torsvik, 2002; Torsvik and Cocks, 2005;
Nawrocki et al., 2007). The geotectonic prov e nance of the Ma³opolska Block is still un clear, but some au thors con sider it as a Gond wana-de rived ter rain (Belka et al., 2000), oth ers as a prox i mal ter rain that de tached from Baltica be fore the Or do vi - cian (Dadlez et al., 1994; Narkiewicz, 2002). South ern part (Kielce Re gion) of the Holy Cross Mts. is con sid ered a north ern part of the Ma³opolska Block.
Or do vi cian/Si lu rian bound ary de pos its de scribed in this pa - per come from the Bardo Stawy and Zalesie Nowe sec tions that are lo cated in the Bardo Syncline (Fig. 1B). The Bardo Syncline is a small tec tonic unit, ~16 km long and 2–5 km wide, ori ented in a NW–SE di rec tion. This Variscan struc ture is com posed of Or do vi cian and Si lu rian de pos its trun cated and con form ably over lain by De vo nian sed i ments (Fig. 1B). These Pa leo zoic de - pos its are blan keted largely by a thick Qua ter nary cover. The out line of the Syncline is un der lined by diabase in tru sions that fol low the bound ary be tween Si lu rian grap to lit ic shales and greywackes.
The Or do vi cian/Si lu rian bound ary in ter val of the Bardo Stawy sec tion com prises a com plete suc ces sion of de pos its from the up per most mucronata, ?persculptus, ascen - sus–acuminatus and vesiculosus biozones (Masiak et al., 2003 and Fig. 2) and ex hib its a con form able and grad ual tran si tion from sandy through silty to clayey de pos its, which is ac com pa - nied by a grad ual change of col our of the rock from light to dark.
This up per most Or do vi cian–low er most Si lu rian suc ces sion in - di cates a depositional en vi ron ment that be came ox y gen-poor dur ing the post-gla cial sea level rise.
De pos its in the Zalesie Nowe sec tion are di vided into lo cal for ma tions (Bednarczyk, 1971, 1981). The top most part of the Zalesie For ma tion (Fig. 3) is com posed of marls and dark red and grey-green ish marly shales that grade con se quently into dark grey shales interbeded by lydites – called the Bardo Beds.
The lower strati graphic unit (up per most Zalesie Fm.) – the so called Dalmanitina Beds – is well-doc u mented by the tri lo bite Mucronaspis mucronata as lat est Ashgill (Kielan, 1956), point - ing to a Hirnantian age. The lat est in ves ti ga tion of graptolite fauna (Kremer, 2001) in di cates a Late Or do vi cian (Ashgill) – Early Si lu rian age for the Bardo Beds at Zalesie Nowe.
A full lithological de scrip tion of both sec tions (Bardo Stawy and Zalesie Nowe) is pre sented in Masiak et al. (2003) and Mustafa et al. (2015). The li thol ogy and lithostratigraphy of the Lower Si lu rian de pos its in the Bardo Stawy sec tion was pre - sented by Trela and Salwa (2007).
BALTICA (BALTIC DEPRESSION)
The Bal tic De pres sion was de vel oped in the south west ern part of the Baltica palaeocontinent (now a days East Eu ro pean Plat form) and was placed ~30°S lat i tude dur ing the Late Or do - vi cian (Cocks and Torsvik, 2002; Torsvik et al., 2002; Torsvik and Cocks, 2005; Nawrocki et al., 2007). Delabroye et al.
(2011a) call this area the Livonian Ba sin af ter Kaljo et al. (2008;
Livonian tongue of the cen tral Baltoscandian Fa cies Belt).
Baltica was geo graph i cally lo cated at low south erly lat i tudes dur ing the Early Or do vi cian and drifted slowly north ward. In Late Or do vi cian/Early Si lu rian times, the East Eu ro pean Plat - form oc cu pied sub trop i cal and equa to rial south ern lat i tudes (Fig. 2).
The up per most Or do vi cian and low er most Si lu rian de pos its in the Bal tic De pres sion have been iden ti fied in nu mer ous bore - holes. In this re gion, all fa cies belts of the Bal tic Ba sin are rec - og nized – from the basinal fa cies in the west, through the slope, deep- and shal low-neritic, to the on shore fa cies in the east (see Podhalañska, 2009).
The ma te rial in ves ti gated in this study co mes from bore - holes sit u ated in the £eba area and cor re sponds to a deep neritic part of the sea, a south ward ex ten sion of the Scanian confacies belt (see Podhalañska, 2009). The west ern bor der of the Bal tic De pres sion is the TESZ and Koszalin–Chojnice Zone, which con tain deep-wa ter lithofacies.
The up per most Or do vi cian strata in the Bal tic De pres sion are gen er ally de vel oped as shaly-cal car e ous, cal car e ous and cal car e ous-dolomitic, and shaly glauconitic de pos its, though the lithofacies dif fer be tween the west ern and east ern parts.
The lithological de scrip tions of some bore hole sec tions can be found, e.g., in Tomczyk (1962), Tomczykowa (1964), Bednar - czyk (1968, 1971, 1996a, 1998, 1999), Podhalañska (1980, 1999, 2003a, b, 2009), Przyby³owicz (1980), Bednarczyk et al.
(1996), Modliñski and Szymañski (1997, 2008), Podhalañska and Modliñski (2006).
Sed i men ta tion across the Or do vi cian/Si lu rian bound ary in the bot tom de pres sions took place prob a bly with out a break and the fa cies changes were grad ual. Non-de po si tion, sub ma - rine ero sion and mi nor hi a tuses took place on bot tom el e va tions and the transgressive un con formity sur face (e.g. in the Koœcierzyna IG 1 sec tion) sep a rates the partly eroded Hirnantian dark grey marls and sand stones con tain ing the Hirnantia fauna from the lam i nated deeper ma rine Rhuddanian shales with graptolites of the acuminatus Biozone.
In the bound ary beds, abun dant ma rine fau nas are well-doc u mented: graptolites (Podhalañska and Modliñski, 2006; Podhalañska, 2009), trilobites and in ar tic u late brachi o - pods (Bednarczyk, 1968; Modliñski, 1988), ostracods, cono - donts (Nehring, 1969; Bednarczyk, 1998), and the Hirnantia fauna (Podhalañska, 1980, 1999, 2003b, 2009).
The to tal thick ness of the up per most Or do vi cian (Ashgill Se ries) in the Bal tic De pres sion var ies from 3.5 to 70 m (Modliñski and Szymañski, 1997). A palaeothickness map of the Mid dle–Up per Or do vi cian de pos its is shown in Modliñski et al. (1999) and Modliñski (2010). The to tal thick ness of the low - er most Si lu rian (Llandovery) var ies from 20 to 70 m (Modliñski et al., 2006), and in creases from east to west.
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KOSZALIN–CHOJNICE ZONE
The Koszalin–Chojnice Zone (Fig. 1A) is placed on an epi-Cal edo nian plat form (Karnkowski, 2008: fig. 2 therein), on the bor der of the SW mar gin of the East Eu ro pean Plat form and Avalonia (e.g., Belka et al., 2000; Poprawa, 2006; Podhalañska and Modliñski, 2006). The Koszalin–Chojnice Zone is re garded as a Cal edo nian front of col li sion de for ma tion (¯elaŸniewicz et al., 2011: fig. 5).
The crys tal line base ment of the Or do vi cian strata in the Koszalin–Chojnice Zone is sup posed to be an Early Pa leo zoic terrane de rived from Gond wana, de scribed as East ern Avalonia (Tait et al., 1997; Pha raoh, 1999; Jaworowski, 2000;
Wrona et al., 2001; Samuelsson et al., 2002). Early Pa leo zoic de pos its of this zone con tain bi otic as sem blages of both Bal tic and Avalonian or i gin (Podhalañska and Modliñski, 2006;
Torsvik and Cocks, 2013).
Dur ing the move ment of Avalonia to wards Baltica, in Or do - vi cian and Si lu rian times, the Koszalin–Chojnice Zone was the prox i mal part, whereas the Bal tic Ba sin was the dis tal part of the same ba sin. The dis tance be tween them was rel a tively close, but they were im me di ately ad ja cent (Krzemiñski and Poprawa, 2006).
The Pa leo zoic sed i men tary cover of the Koszalin–Chojnice Zone in cludes Or do vi cian de pos its iden ti fied in many bore holes that were drilled mainly in the 1960s and 1970s (Tomczyk, 1968; Modliñski, 1968, 1978, 1987; Bednarczyk, 1974; Dadlez, 1978, 1982a, b, 1993, 2000).
The Mid dle–Up per Or do vi cian and Si lu rian de pos its form the Older Pa leo zoic se quence. This se quence, com posed mainly of shales (dark grey and grey clayey-muddy sed i ments with scarce sandy, dolomitic and si der it ic in ter ca la tions – e.g., Krzemiñski and Poprawa, 2006), is in tensely folded and un con - form ably over lain by the youn ger Pa leo zoic se quence.
The strata above the Or do vi cian lo cally at tain 4000 m in thick ness. The Up per Or do vi cian in the Koszalin–Chojnice Zone is doc u mented (Podhalañska and Modliñski, 2006) by graptolites only for the Sandbian (Caradocian). Other groups of fos sils, such chitinozoans, prasinophytes and var i ous trace fos - sils, also frag ments of brachi o pods and trilobites, oc cur spo rad i - cally (Bednarczyk, 1974; Wrona et al., 2001; Podhalañska, 2007). Palynological data based on acritarchs in di cates a Llanvirn–Caradoc age of the Or do vi cian de pos its of the Pom er - a nian Caledonides (Szczepanik, 2000).
The lat est strati graphic di vi sion of the up per most Or do vi - cian is based on a cor re la tion with the Polskie £¹ki PIG 1 bore - hole and other bore holes. The over ly ing de pos its are Sandbian in age, and are strongly de formed and faulted. A ten ta tive strati - graphic sub di vi sion is based on lithofacies stud ies (Podhalañ - ska and Modliñski, 2006).
Llandovery de pos its are rep re sented by deep-ma rine siliciclastic sed i ments most fre quently con tain ing sed i men tary struc tures re lated to sea cur rent ac tiv ity (Teller and Korejwo, 1968; Teller, 1974; Podhalañska and Modliñski, 2006). The strata doc u mented in the Toruñ 1 bore hole are Si lu rian in age (lower Llandovery, Rhuddanian; Dadlez, 1982b; Tomczyk, 1987). Cur rently, it is not pos si ble to de fine the de tailed stra tig - ra phy due to poor pres er va tion of the rock and tec tonic de for - ma tion. The orig i nal thick ness of the strongly folded Llandovery de pos its (Lutom 1 and Toruñ 1 sec tions) is >400 m (Poprawa, 2006).
The macrofaunal as sem blage found in the Llandovery de - pos its was dom i nated by graptolites and rare in ar tic u late brachi o pods. The fre quency of graptolites is low. Among microfossils, the pres ence of acritarchs, prasinophytes and
chitinozoans was doc u mented by Teller (1974), Jachowicz (2000), Wrona et al. (2001) and Podhalañska and Modliñski (2006). Nu mer ous tec toni cally de formed biserial graptolites were found in black shale in the Toruñ 1 sec tion at a depth of 5298.0 m (Podhalañska and Modliñski, 2006). They in di cate the pres ence of Rhuddanian de pos its, which suc ceed the strongly folded Up per Or do vi cian rocks.
MATERIAL AND METHODS
MATERIAL
The palynological in ves ti ga tion pre sented in this pa per in - cludes data from 73 sam ples col lected and stud ied over the past few years – palynological data, es pe cially palynomorph fre quency, were ob tained as an ad di tional re sult of stud ies on stra tig ra phy by the In sti tute of Geo log i cal Sci ences PAS in War saw. Thus, only acritarchs were counted in some cases (Zalesie Nowe and Koœcierzyna IG 1), but gen er ally acritarchs and prasinophytes were counted for their fre quency.
Thirty-seven sam ples from the Holy Cross Moun tains (Fig. 1B) yield both well-pre served ma te rial – yel low to brown acritarchs and yel low prasinophytes in the Bardo Stawy sam - ples – and less well-pre served dark brown to black palynomorphs in the Zalesie Nowe sam ples. De tailed de scrip - tion of sam ples is pub lished in Masiak et al. (2003) and shown in Fig ures 2 and 3 in this pa per.
The ma te rial from the East Eu ro pean Plat form (Baltica palaeocontient, Bal tic De pres sion) con sists of sam ples from three bore holes: Bia³ogóra 1, Koœcierzyna IG 1 and £eba 8 (Fig. 1A). Thirty-two sam ples from dark grey shales were taken, which span a strati graphic in ter val equiv a lent to the up per most Hirnantia beds of the mucronata tri lo bite Biozone, the ascensus graptolite Biozone (Early Si lu rian, Rhuddanian), and the low er - most part of the suc ceed ing acuminatus graptolite Biozone (Si - lu rian).
The ma te rial stud ied for palynology in the Koszalin–Choj - nice Zone com prises four sam ples from the Toruñ 1 bore hole (Fig. 1A). All sam ples are from dark grey shale. This in ter val prob a bly spans the up per most part of the Hirnantia beds (mucronata tri lo bite Biozone) to the Llandovery (Rhuddanian) un di vided.
The ma te rial from the East Eu ro pean Plat form and the Koszalin–Chojnice Zone is poorly pre served, mostly in com - plete and dark brown to black in col our.
Pres er va tion in some sam ples is too poor to dis tin guish the acanthomorphic acritarchs with a short pro cess from prasino - phyte al gae. In other sam ples, how ever, the sphaeromorphs are suf fi ciently well-pre served – es pe cially the ex is tence of thin-walled forms – to in di cate that the lack of acritarchs is prob - a bly pri mary and not the re sult of pres er va tion. Ob vi ously, it can not be ex cluded that some spec i mens were de stroyed dur - ing post-sed i men ta tion pro cesses.
There are few sam ples from the Toruñ 1 bore hole be cause of long non-cored in ter vals. Gen er ally, all palaeontological data from bore holes of the Koszalin–Chojnice Zone are scarce and poor, so the au thors de cided to pub lish even these mod est data.
The po si tion of in di vid ual sam ples taken from bore holes is mea sured di rectly from the core depth, not from well logs. All slides are housed at the De part ment of Stra tig ra phy and Palaeo ge ogra phy, In sti tute of Geo log i cal Sci ences, PAS (ING PAN) War saw, Po land.
PALYNOLOGICAL METHODS
All sam ples were sub jected to a stan dard palynological lab - o ra tory treat ment (Wood et al., 1996). The sam ples were used for quan ti ta tive anal y sis of as sem blage sim i lar ity. In this anal y - sis, the or ganic res i dues ob tained af ter com plete dis so lu tion of 5 g of rock were sup ple mented to a vol ume of 4 ml with methyl al co hol. From each sam ple, three slides were made; and for each slide, 0.5 ml of well-mixed so lu tion was ana lysed for palynomorphs. For each sam ple level, the arith me tic mean was cal cu lated from three slides. The same method was used in Masiak et al. (2003).
RESULTS
MA£OPOLSKA BLOCK
Palynological data from the sec tions and bore holes are dif - fer ent. The rich est and most di verse as sem blages are from the Early Si lu rian of the Bardo Stawy sec tion (Fig. 2). This sec tion is the best-doc u mented ac cord ing to palynological and grap to lit ic as sem blages (Masiak et al., 2003) and in our in ves ti ga tion is treated as a bench mark for com par i son with other sec tions.
The lat est Or do vi cian palynological as sem blage in the Bardo Stawy sec tion in cludes only long-rang ing acritarchs, mainly of sim ple mor phol ogy. The acritarch fre quency is poor to mod er ate (sam ples BS. 1 to BS. 4) – from 0 to 20 spec i mens per sam ple. The Or do vi cian/Si lu rian bound ary in the Bardo Stawy sec tion (BS. 5) is marked by a con sid er able de crease of acritach fre quency – the sam ple is bar ren. The fre quency and di ver sity in crease in the lower part of the ascensus–acuminatus graptolite Biozone of the Early Si lu rian (BS. 6–BS. 13): a sig nif i - cant peak in both di ver sity and fre quency is ob served in sam ple BS. 9 (up to 280 spec i mens per slide). In the Bardo Stawy sec - tion, the max i mum acritarch fre quency is at tained in the lower part of the vesiculosus graptolite Biozone, al though the gen era and spe cies di ver sity do not change (sam ple BS. 19). This change in acritarch as sem blages does not ap pear to be re lated to li thol ogy. For de tailed de scrip tions of palynological as sem - blages see Masiak et al. (2003). Apart from acritarchs, there are many sphaeromorphs, such as Leiosphaeridia spp. re garded as prasinophytes (Le Hérissé, 1989), as well as prasinophytes from two gen era: Cymatiosphaera and Pterospermella sp.
(sam ples BS. 17–BS. 19).
The num ber of prasinophyte spec i mens was counted in sam ples BS. 1 to BS. 18. Sam ples BS. 1 to BS. 3 con tain no spec i mens. From BS. 4 to BS. 5, the num ber of spec i mens in - creases to 260 spec i mens per slide. The next peak in the fre - quency is noted in sam ple BS. 9 (1300 spec i mens per slide) and then a de creas ing trend is ob served. A min i mum is noted in BS. 16 (no spec i mens). Start ing from sam ple BS. 17, the num - ber of spec i mens rap idly in creases and the max i mum fre - quency is noted in sam ple BS. 18 – 13,000 spec i mens.
In the Zalesie Nowe sec tion (Fig. 3), the trend in microphytoplankton di ver si fi ca tion is sim i lar, but the over all di - ver sity is al ways lower than in the Bardo Stawy sec tion, which could be con nected with dif fer ences in li thol ogy. The main dif - fer ence here is the poorly de fined O/S bound ary be cause of lack of in dex graptolite.
A dis tinct drop in acritarch and prasinophyte fre quency is ev i dent in the O/S bound ary zone in the Zalesie Nowe sec tion within the in ter val of clayey shales (Z12–Z13). This change is sim i lar to the de crease in fre quency in the Bardo Stawy sec tion (?persculptus – low er most part of the ascensus–acuminatus biozones). This bound ary zone in volves most prob a bly the
?persculptus graptolite Biozone and the low er most part of the ascensus–acuminatus Biozone and is de fined by the oc cur - rence of a dis tinct drop in acritarch fre quency, sim i lar to that of the Bardo Stawy pro file. Re gard ing the fre quency and di ver sity of microphytoplankton, the peak ob served in sam ple Z. 15 may be at the same level as that of sam ple BS. 9 in the Bardo Stawy sec tion. De tailed de scrip tion of the Zalesie Nowe sec tion is pub lished in Masiak et al. (2003) and Mustafa et al. (2015).
Microphytoplankton from the de scribed sec tions of the HCM is gen er ally well pre served, yel low to pale brown in col our.
BALTICA (BALTIC DEPRESSION)
Microphytoplankton as sem blages of the Bia³ogóra 1, Koœcierzyna IG 1 and £eba 8 bore holes are gen er ally poor and poorly pre served, dark grey to black in col our, and surficially cor roded (in con trast to those from the south ern part of the Holy Cross Mts., which are much clearer), which is caused by dif fer - ent ther mal ma tu rity.
Range charts of the most im por tant gen era and spe cies are shown in Fig ures 4–6.
BIA£OGÓRA 1 BOREHOLE.
De tailed data from the Bia³ogóra 1 bore hole is shown in Fig - ure 4.
The low er most palynological sam ple from the up per most Or do vi cian (Hirnantia beds) in the Bia³ogóra 1 bore hole (sam - ple B1.8) is bar ren. The first change in di ver sity and fre quency of microphytoplankton oc curs in the up per part of the Hirnantian in sam ple B1.7. The as sem blage is poorly di verse and com - prises mainly long-rang ing taxa, such as Micrhystridium sp., Multiplicispaeridium sp., Veryhachium sp. and im por tantly the Up per Or do vi cian in dex taxa of Baltisphaeridium sp., Ordovicidium sp. and Orthosphaeridium sp. The sam ple con - tains prasinophytes (Leiosphaeridia spp.) com pa ra ble in num - ber to the acritarchs.
The slide for sam ple B1.6 com prises 46 spec i mens of acritarchs rec og niz able mainly at the ge neric level:
Baltisphaeridium sp. (prob a bly re de pos ited), Diexallophasis sp,. Micrhystridium sp., Multiplicisphaeridum sp. and Oppilatala sp. This sam ple con tains a very large num ber of prasinophytes (Leiosphaeridia spp.) up to 200 spec i mens per slide.
The fre quency in the next sam ple (B1.5) is de creas ing with only nine leiosphaeres (Prasinphyceae) and two un rec og niz - able acanthomorphs. Sam ple B1.4 is more abun dant – it con - tains 33 spec i mens per slide. Some new spe cies ap pear:
Tylotopalla caelamenicutis, T. deerlijkianum, T. guapa and
?Oppilatala sp. The sam ple con tains a huge num ber of prasinophytes (Leiosphaeridia spp.) – 880 spec i mens per slide.
In fol low ing two sam ples – B1.3 and B1.2 that rep re sent the up per part of the ascensus Biozone, the abun dance of acritarchs goes down to <20 spec i mens per slide (B1.3 – 17 spec i mens, B1.2 – 10), but the num ber of prasinophytes is de - creas ing to 230 spec i mens per slide.
The fre quency and di ver si fi ca tion of acritarchs in the sam - ple from the low er most part of the acuminatus Biozone (B1.1) in creases to 43 spec i mens per slide, whilst the fre quency of prasinophytes is still prom i nent – >500 spec i mens.
KOŒCIERZYNA IG 1 BOREHOLE
De tailed data from the Koœcierzyna IG 1 bore hole is shown in Fig ure 5. The palynological sam ples from the up per most Or - do vi cian (Hirnantia beds) in this bore hole (sam ples K21–K15)
elo herob 1 arógo³aiB eht ni ycneu qerf hcratirca + etyhponisarp dna hcratirca dna ,trahc egnarhcratirca dna yg oloh til dei fil pmiS .4 .giF ees snoi ta nal pxe roF2 er ugiF
are bar ren. The first change in di ver sity and fre quency of microphytoplankton oc curs in the up per Hirnantian in sam ple K13. The as sem blage is poorly di ver si fied and com prises mainly long-rang ing taxa, such as Micrhystridium sp. and Solisphaeridium sp., and the Up per Or do vi cian in dex taxon Baltisphaeridium sp.
The slide for sam ple K10, which is the first Si lu rian sam ple, com prises 30 spec i mens of acritarchs rec og niz able mainly at the ge neric level: Baltisphaeridium sp. (prob a bly re de pos ited), Micrhystridium sp., Multiplicisphaeridum cf. irregulare, Veryha - chium trispinosum, Tylotopalla sp. and Solisphaeridium sp.
The fre quency in the next sam ple (K9’M) is de creas ing, only a few leiospheres and two acanthomorphs oc cur (Multiplici - sphaeridium sp. and Tylotopalla sp.). The as sem blages in sam -
ple K8M are more abun dant – com pris ing 17 spec i mens per slide. Some new spe cies ap pear: Ammonidium sp., Diexallophasis sp., Elektoriskos sp. and Tylotopalla deerlij - kianum.
In the fol low ing sam ple – K4,6M, the abun dance of microphytoplankton de creases. The fre quency in the next sam ple K2 in creases to 18 spec i mens per slide. The as sem blage is poorly di ver si fied and con tains only long-rang ing taxa: Very hachium trispinosum, Multiplicisphaeridium sp. and Tylotopalla sp.
In the two last sam ples K1 and K3M, the fre quency dra mat i - cally de creases (2 spec i mens per slide). In K1, two new taxa ap pear: Diexallophasis robustospinosa and Navifusa sp.
Prasinophytes were not counted (see ex pla na tions in chap - ter Ma te rial and meth ods).
elo herob 1 GI anyzreicœoK eht ni ycneu qerf hcratirca dna ,trahc egnarhcratirca dna yg oloh til dei fil pmiS .5 .giF ees snoi ta nal pxe roF2 er ugiF
£EBA 8 BOREHOLE
De tailed data from the £eba 8 bore hole is shown in Fig ure 6.
Microphytoplankton is very poorly pre served (dark brown and black) in the bore hole. Only three recogniszable taxa oc cur.
Most acanthomorphs are in de ter mi nate even to the ge neric level, and there are also some Leiosphaeridia. The most pro - duc tive sam ple in terms of acritarch taxa recognizability is L8.12 (persculptus? graptolite Biozone), while sam ple L8.9 (prob a bly the up per most part of the ascensus + acuminatus graptolite Biozone) is richer in both acritarchs and prasinophytes – with 12 spec i mens per slide.
KOSZALIN–CHOJNICE ZONE – TORUÑ 1 BOREHOLE
De tailed data from the Toruñ 1 bore hole is shown in Fig ure 7.
The palynological as sem blage in the sam ples from the Up - per most Or do vi cian (Hirnantia beds) of the Toruñ 1 bore hole (sam ples T.1.5 and T.1.4) is di verse and mod er ately abun dant (up to 18 spec i mens per slide). It com prises mainly long-rang - ing taxa. Al most all the above-listed, ex cept Deunffia sp. and Pterospermella sp., oc cur in sam ple T.1.3 (low er most Si lu rian).
The abun dance dra mat i cally de creases through the in ves ti - gated in ter val to three spec i mens per slide in T.1.3, and to two spec i mens in sam ple T.1.2.
COMPARISON OF DATA FROM POLAND WITH OTHER REGIONS
A ma jor prob lem af fect ing the anal y sis of acritarch dis tri bu - tion across the Or do vi cian–Si lu rian bound ary is the lithofacies con trol on pres er va tion of or ganic-walled microfossils.
Palynomorphs, such as acritrachs and prasinopytes, pre fer spe cific, char ac ter is tic fa cies of en vi ron ments with prev a lence of fine-clastic sed i men ta tion. Based on this fact, in the other world lo cal i ties of the O/S bound ary, the pos si bil ity of fol low ing the trend of fre quency and di ver sity of acritarch as sem blages is strait ened be cause of lack of rel e vant fa cies and con ti nu ity of de pos its (Fig. 8).
The Bardo Stawy, Zalesie Nowe, Bia³ogóra and £eba sec - tions are unique be cause of sed i men tary con ti nu ity and palynological yields across the O/S bound ary, as well as good in de pend ent strati graphic con trol pro vided by graptolites.
In the fol low ing sec tion, we com pare the pres ent re sults with pre vi ously pub lished sim i lar stud ies, as con ducted in a dif - fer ent palaeogeographic po si tion, re gard ing also the bathymetry and de pend ence of acritarchs and prasinophytes on sea depth (Al-Ameri, 1983).
Fig. 6. Sim pli fied li thol ogy and acritarch range chart, and acritarch and prasinophyte + acritarch fre quency in the £eba 8 bore hole
For ex pla na tions see Fig ure 2
LOW TO MIDDLE LATITUDES OF THE SOUTHERN HEMISPHERE
Con sid er ing the palaeo geo graphi cal po si tion of the main land masses in Late Or do vi cian times (Fig. 1C, D), Baltica oc cu - pied sub trop i cal lat i tudes be tween an equa tor and 30°S-lat i tude (Torsvik and Cocks, 2004). This con ti nent was sep a rated from Gond wana (on the South Pole) by the Rheic Ocean. Smaller blocks, such as Avalonia, the Bo he mian Mas sif and Ma³opolska Block, oc cu pied ~30°S-lat i tude in the Late Or do vi - cian (Cocks, 2000; Torsvik and Cocks, 2005). All these lo cal i - ties were placed more or less in the same cli ma tic zone. Dif fer - ences within the microphytoplankton con tent of de pos its may re sults from en vi ron men tal depositional con di tions, such as bathymetry, in flu ence of oce anic cur rents, etc.
BALTICA
Apart from the Pol ish lo cal i ties de scribed above, other palynological data co mes from Es to nia – from the Rapla bore - hole in north ern Es to nia and the Valga 10 bore hole in south ern Es to nia.
At Rapla, where the prob lem of fa cies con trol on pres er va - tion of or ganic-walled microfossils is ev i dent in sec tions span - ning the O/S bound ary (Uutela and Tynni, 1991), shal low-wa ter car bon ate plat forms de vel oped dur ing the Late Or do vi cian. The pres ence of dis con ti nu ity sur faces within the bound ary in ter val and lack of pre cise strati graphic con trol are ad di tional prob lems for this sec tion. In the up per most Ashgill (Hirnantian) of the Rapla bore hole (Uutela and Tynni, 1991), 40 acritarch and prasinophyte spe cies were re corded, of which 29 cross the O/S Fig. 7. Sim pli fied li thol ogy and acritarch range chart, and acritarch and prasinophyte + acritarch fre quency
in the Toruñ 1 bore hole For other ex pla na tions see Fig ure 2
noi tces ywatS odraB eht ni sevruc ycneu qerf hpromonylap dna COT fo no s ira pmoC .8 .giF A ;ycneu qerfhcratirca dna etyhponisarp –B re tfa( yr adnuob noi ti snartnai ru liS/nai ci vo drO eht ssorca eg atne crep COT fo sdnert la cihpar gitarts –5102 ,.la te afatsuM)
bound ary. As sem blages from the Lower Si lu rian strata are im - pov er ished, with only four acritarch spe cies re corded.
In terms of the abun dance, Micrhystridium sp. and Lophosphaeridium sp. are the dom i nant taxa. Other taxa, such as Baltisphaeridium sp, Gorgonisphaeridium sp, Leiosphaeri - dia spp., Stellechinatum sp., Multiplicisphaeridium sp., Tasmanites sp., Cymatiosphaera sp., Pulvinosphaeridium sp.
and Estiastra sp. are rare.
Gen er ally, the trend in the fre quency of gen era and spe cies is de creas ing from the up per most Ashgill (mid dle Pirgu, Re - gional Stage) to the low er most Si lu rian (low er most Juurgu, Re - gional Stage; Kaljo et al., 1996: ta ble 1 and fig. 3).The O/S bound ary in the Rapla bore hole is char ac ter ized by a mi nor dis - con ti nu ity sur face. In the Porkuni Re gional Stage (up per most Ashgill), acritarchs are gen er ally ab sent due to both sec ond ary dolomitization and the oc cur rence of high-en ergy sed i ments of reef fa cies (Kaljo et al., 1996).
Fol low ing the trend in the in creas ing/de creas ing num ber of gen era and spe cies, it is pos si ble to com pare data from the Rapla bore hole with the Pol ish data.
In gen eral, the up per most Or do vi cian as sem blages from Po land are less nu mer ous than those in Rapla, but the low er - most Si lu rian ones are more. In the Pol ish pro files, there are no typ i cal shal low-wa ter car bon ate plat form gen era, such as Estiastra, Pulvinosphaeridium and Schismatosphaeridium.
More in for ma tion co mes from de tailed stud ies on palynological in ves ti ga tions from the Valga 10 drill core (Delabroye et al., 2011a). The lat est Or do vi cian/ear li est Si lu rian de pos its from this bore hole of south ern Es to nia rep re sent tran - si tional en vi ron ments of the Livonian Ba sin – a tran si tional ba - sin be tween north Es to nian car bon ate shelves and deeper-wa - ter sed i ments of the Scan di na vian Ba sin.
Palynological ma te rial from the Valga 10 drill core rep re - sents shal lower-wa ter de pos its than those from the Bal tic De - pres sion and the south ern part of the Holy Cross Mts. The in - ves ti gated pro file of the Valga 10 bore hole in volves a lon ger time in ter val – the lat est Katian and Hirnantian (Or do vi cian) and the ear li est Si lu rian (Delabroye et al., 2011a). The cor re la tions with other pro files are hin dered: chitinozoan biozones have not been iden ti fied; prob a bly some re de pos ited as sem blages oc - cur (Delabroye et al., 2011a and ref er ences therein).
De pos its of the O/S bound ary from Gotland are known from the Nar bore hole. They oc cur at sim i lar depths to those from the Valga 10. Stra tig ra phy of this in ter val in the Nar bore hole dur ing acritarch and prasinophyte in ves ti ga tion was in com plete and showed the ex is tence of a hi a tus at the O/S bound ary, which in - cluded the acuminatus Zone (Le Hérissé, 1989).
It is dif fi cult to com pare quan ti ta tive data; how ever, table 2 in Le Hérissé (1989) shows the ex is tence of a tax o nom i cally di - verse (14 taxa) as sem blage.
The lat est Katian and Hirnantian palynological as sem - blages were stud ied both in the Bal tic De pres sion (Stempieñ-Sa³ek, 2011) and the Holy Cross Mts. (Trela and Szczepanik, 2009). In the Szumsko Kolonia bore hole, drilled in the Bardo Syncline, the up per Hirnantian acritarch as sem - blages are sim i lar to those from Zalesie and Bardo Stawy, al - though, the di ver sity and fre quency are higher. Par tic u larly in - ter est ing is the oc cur rence of taxa from the peri-Gondwanan acritarch palaeoprovince (Trela and Szczepanik, 2009).
The sam ple from up per Hirnantian de pos its of the Szumsko Kolonia bore hole con tains fre quent and di ver si fied as sem - blages dom i nated by the Veryhachium taxa (70%). This sam ple co mes from older de pos its than those de scribed in the Bardo Stawy and Zalesie Nowe sec tions, and the palynological as - sem blages are com pa ra ble to the co eval ones from the Valga 10 bore hole, al though the com po si tion is dif fer ent (dom i nance
of Veryhachachium ver sus cryptospores). For a full list of microphytoplankton as sem blages see Delabroye et al. (2011a) and Trela and Szczepanik (2009).
The up per most Hirnantian acritarch as sem blages in the Bardo Stawy and Zalesie Nowe sec tions are sim i lar to those from the Szumsko Kolonia and Valga 1 bore holes, al though they are poorer.
Com par i son of the ear li est Si lu rian acritarchs is dif fi cult be - cause there is only one pro duc tive Rhuddanian sam ple from the Valga 10 drill core. This sam ple (Valga 10) from the low er - most Si lu rian con tains a small and poorly di ver si fied as sem - blage dom i nated by one morphotype of Leiosphaeridia. Such an as sem blage is sim i lar to those from the Bardo Stawy sec tion (Fig. 2). The same peak in the fre quency of Leiosphaeridia (whereas acritarchs are rare) is ob served in sam ple BS. 5, just above the O/S bound ary. A sim i lar sit u a tion is ob served in sam - ple Z. 14 in the Zalesie Nowe sec tion (Fig. 3). Other (youn ger) Si lu rian sam ples from the south ern part of HCM (BS. 6–BS. 20) show a di ver si fi ca tion trend in the palynological as sem blage.
Such a di ver si fi ca tion of acritarch as sem blage dis agrees with a gen eral state ment in Delabroye et al. (2011a: 33) that ex - tremely low di ver sity seems to be a gen eral fea ture of phytoplanktonic as sem blages of the be gin ning of the Si lu rian, dom i nated mainly by sphaeromorphs and en vi ron men tally tol - er ant acritarchs. More di verse acritarch as so ci a tions de vel oped at the Rhuddanian/Aeronian tran si tion (Duffield and Legault, 1981; Mar tin, 1989; Le Hérissé, 2000; Vecoli, 2008).
In our opin ion, this is prob a bly true for acritarch as sem - blages from de pos its rep re sent ing rather shal lower fa cies, sim i - lar to those in south ern Es to nia (Valga 10), as well as deep ba - sin fa cies, e.g. from north ern Po land (Bal tic De pres sion):
Bia³ogóra 1 and £eba 8.
Acritarchs in the Bardo Stawy (S-HCM) sec tion come from de pos its con sid ered (prob a bly) deep-wa ter sed i ments, but not so deep as in north ern Po land. In the Bardo Stawy sec tion, di - ver si fi ca tion of the acritarch as sem blage starts in de pos its of the acuminatus–ascensus Biozone. Ad di tion ally, de pos its from the O/S in ter val of the Zalesie Nowe sec tion come from shal - lower en vi ron ments (Bednarczyk, 1996b; Trela, 2005, 2006;
Trela and Szczepanik, 2009).
While study ing the dis tri bu tion of palynomorphs taxa, a clear de pend ence on bathymetry is ob served. The dom i nance of prasinophytes (leiosphaeres) and crypyospores is char ac ter - is tic for shal lower depths. In deeper wa ter, acanthomorphs are more abun dant; they dom i nate in the as sem blage. In the deep - est en vi ron ments, the dom i nance of leiosphaeres re turns, al - though acantomorphs are nu mer ous, but not as abun dant as in shal lower ones. Ad di tion ally, giganteus acritarchs oc cur in car - bon ate de pos its (Le Hérissé, 1989). Such a pat tern is gen er ally com pat i ble with a bathymetric model by Dorning (1981) and Al-Ameri (1983).
LAURENTIA
At the Or do vi cian/Si lu rian tran si tion, Anticosti Is land (Qué - bec, east ern Can ada) was lo cated at low to in ter me di ate palaeolatitudes (15–30°S) on the east ern mar gin of Laurentia (Torsvik and Cocks, 2005; Nawrocki et al., 2007).
The up per most Or do vi cian–low er most Si lu rian de pos its of Anticosti Is land, Qué bec, are rep re sented by a car bon ate se - quence of cal car e ous shales and interbedded fossiliferous lime stones, shales and bioherms that were de pos ited in shal low to deep subtidal en vi ron ments. In dif fer ent sec tions of Anticosti Is land, the O/S bound ary is drawn within dif fer ent types of sed i - ments (Barnes, 1988).
Very in ter est ing palynological data on Hirnantian acritarchs from Anticosti were pub lished by Delabroye et al. (2011b).
These au thors ob served a post-cri sis, low di ver sity acritarch as - sem blage dom i nated by large forms typ i cal of car bon ate plat - forms, as well as long-rang ing taxa and cryptospores (land-de - rived), while typ i cal Or do vi cian taxa (Ordovicidium spp., Baltisphaeridium spp., Peteinosphaeridium spp.) were not re - corded.
At Anticosti, the O/S bound ary sec tions are char ac ter ized by scant biostratigraphic con trol (lack of graptolite fauna) and the pres ence of paly no logi cal ly bar ren strata (e.g., bioherm beds above the sys temic bound ary; Mar tin, 1988). This hin ders the anal y sis of microphytoplankton di ver sity trends.
CHINESE BLOCK
In the Zhejiang Prov ince of South China, con tin u ous Or do - vi cian–Si lu rian tran si tional strata are char ac ter ized by low di - ver sity and un fa vour able pres er va tion of palynomorphs. There is no clear change in the acritarch palynoflora across the Or do - vi cian/Si lu rian bound ary. The pres ence of abun dant crypto - spores may sug gest a near-shore ma rine en vi ron ment dur ing lat est Or do vi cian–ear li est Si lu rian times. Acritarchs be come abun dant and well pre served 400 m above the ac cepted Or do - vi cian/Si lu rian bound ary. The in suf fi ciency of de tailed in ves ti ga - tion of microphytoplankton at the O/S bound ary, due to sparse sam pling along the thick sec tion, is the ma jor ob sta cle of this sec tion (Yin and He, 2000).
HIGH PALAEOLATITUDES OF THE SOUTHERN HEMISPHERE
BOHEMIAN MASSIF (PERUNICA)
The high-lat i tude palaeogeograpic po si tion of the Bo he mian Mas sif at the O/S tran si tion, shown by Delabroye et al. (2011a), is ac cepted herein (Fig. 1C). Ac cord ing to that pa per, the Bo he - mian Mas sif is clas si fied as a ter rain ad ja cent to North ern Gond wana.
The Or do vi cian–Si lu rian bound ary in the Prague Ba sin is iden ti fied in two con tin u ous sec tions (Dufka and Fatka, 1993).
Both sec tions are well-dated by means of graptolites (Štroch, 1986). The O/S bound ary in ter val is char ac ter ized there by a tran si tion from flysch-type de pos its into black grap to lit ic shales.
The dark shales with graptolites ap pear al ready be low the base of the ascensus Biozone and they cor re spond prob a bly to the up per most part of the persculptus Biozone (sim i lar sit u a tion to the south ern part of the Holy Cross Mts.). The weak ness of this sec tion for palynological in ves ti ga tions is the ab sence of acritarchs and prasinophytes from the base of the Llandovery.
Acritarchs were re corded only from the green-grey claystone of the up per part of the acuminatus graptolite Biozone – ~1.2 m above the bound ary (Dufka and Fatka, 1993).
GONDWANA
In Saudi Ara bia, well-es tab lished chitinozoan zones could be cor re lated with the graptolite zonation of the Brit ish Stan - dard. The sys temic in ter val – persculptus and the lower part of the acuminatus graptolite biozones – is char ac ter ized by low di - ver sity of microphytoplankton. The or ganic-rich black shales (“Hot Shales”) from the mid dle and up per part of the acuminatus graptolite Biozone are par tic u larly un fa vour able for acritarchs and prasinophytes. Only some cryptospores were found
(Spina, 2015). How ever, it is im por tant to note that this zone is not suf fi ciently in ves ti gated. For full un der stand ing of the re cov - ery pat tern of microphytoplankton, it is im por tant to study the silty in ter ca la tion and clays within the black grap to lit ic shales (Le Hérissé, 2000).
Some in ter est ing data come from the Tt1 bore hole – Ghadamis Ba sin, south ern Tu ni sia (Vecoli et al., 2009). “There is no ev i dence of the ma jor dis con ti nu ity at the Or do vi cian/Si lu - rian bound ary even the se quence is con densed”, as stated by Vecoli et al. (2009), and the Si lu rian be gins from the vesiculosus Biozone. The ascensus–acuminatus Biozone is not pres ent there.
In NE Al ge rian Sa hara (bore hole Nl-2) the pro file is not con - tin u ous across the O/S bound ary and is char ac ter ized by an im - por tant gap com pris ing most of the up per part of the Or do vi cian and the whole Llandovery. This hi a tus caused the Wen lock black shales to over lie di rectly the Hirnantian sed i ments (Paris et al., 2000). Other lo cal i ties (Chad) from north ern Af rica are not well con trolled stratigraphically (Le Hérissé et al., 2013).
MICROPHYTOPLANKTON ABUNDANCE AND TOTAL ORGANIC CARBON
Or ganic mat ter is pri mar ily de fined as de rived from the ac - cu mu la tion and pres er va tion of or ganic com pounds that di rectly or in di rectly come from cells or tis sues of liv ing or gan isms. In the case of shale rocks, the TOC con tent is in flu enced not only by phytoplankton (e.g., acritarchs and prasinophytes in the pres ent study) but also by graptolites, al gae and other or gan - isms.
The to tal or ganic car bon con tent in rocks from the Bardo Stawy sec tion was stud ied by Mustafa et al. (2015). It is very in - ter est ing to com pare the microphytoplankton (prasinophyte and acritarch) abun dance with the TOC curve in the same time in - ter val (Fig. 9). Sim i lar com par i son of TOC and acritarch and prasinophyte abun dance was pre sented by Vecoli et al. (2009), but the Si lu rian in the bore hole from the Ghadamis Ba sin be - gins from the vesiculosus Biozone. In the Bardo Stawy sec tion, the O/S transiton is con tin u ous so that the data ob tained from the ascensus–acuminatus Biozone gives a fuller pic ture of changes on the bound ary of these two sys tems. The prasinophyte and acritarch abun dance and the TOC trend are very sim i lar in the up per most Or do vi cian de pos its (low val ues).
From the be gin ning of the Si lu rian, the TOC and acritarch–prasinophyte curves show a gen eral in creas ing trend, al though both curves dem on strate vari a tions. De vi a tions of both curves are con cor dant in the lower part of the ascensus–acuminatus Biozone, but they are op po site in its up - per part (Fig. 9).
It is very prob a ble that that time an other source of or ganic car bon played a sig nif i cant role. It is highly pos si ble that this role was played by graptolites dur ing the post-Hirnantian gla ci ation bi otic re cov ery phase.
The com par i son of the TOC and microphytoplankton curves was pos si ble only for sed i ments from the Bardo Stawy re gard ing our avail able data. Pre lim i nary data are pro pi tious and it is worth to con tinue such in ves ti ga tions.
CONCLUSIONS
1. Acritarch and prasinophyte as sem blages at the Or do vi - cian/Si lu rian bound ary ap pear to be good in di ca tors of cli ma tic
snoi ger rehto htiw dna loP morf atad fo no s ira pmoC .9 .giF ni sa snoitanalpxe rehtO2 erugiF
changes con nected with deglaciation of Gond wana and as so ci - ated glacioeustatic changes.
2. There are only a few known palynomoprh-yield ing pro - files with a con tin u ous O/S tran si tion. The pro files from Po land, es pe cially from the Holy Cross Mts., are very good for in ves ti ga - tions of global cli ma tic and eustatic events in the lat est Or do vi - cian to ear li est Si lu rian, re corded in palynological as sem blages.
3. Bathymetry played the key role in tax o nomic di ver si fi ca - tion of the microphytoplankton as sem blages, which sup ports the ex ist ing mod els of dis tri bu tion (e.g., Al-Ameri, 1983).
4. Dom i nance of prasinophytes (leiospheres) and crypotospores is char ac ter is tic for shal low-wa ter en vi ron ments.
In deeper wa ter, dom i nance of acanthomorphs is ob served. In the deep est-wa ter zones – mixed as sem blages oc curred. The
model pre sented in Delabroye et al. (2011b) has been ver i fied only in the shal lower en vi ron ments.
5. The anal y sis of acritarch di ver sity trends vs. To tal Or - ganic Car bon (TOC) pro files shows par al lel trends through the sec tion: a low value in the Up per Or do vi cian and an in creas ing trend from the be gin ning of the Si lu rian. Op po site de vi a tions on the TOC and arcitarch–prasinophyte abun dance curves may in di cate that other or gan isms sig nif i cantly con trib uted to the ac - cu mu la tion of or ganic car bon in sed i ments dur ing that time.
Ac knowl edge ments. The authors are greatly in debted to the re view ers: T. Servais (French Na tional Cen tre for Sci en tific Re search) and M. Vecoli (Saudi Ara bian Oil Com pany) for their valu able com ments on our text.
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