Trace el e ment vari a tions as a proxy for re con struc tion of palaeoenvironmental changes dur ing the Late Aeronian fau nal and car bon iso tope per tur ba tions:
new data from the peri-Gondwanan region
Jan PAŠAVA1, *, Jiøí FRÝDA1, 2 and Petr ŠTORCH3
1 Czech Geo log i cal Sur vey, Klárov 3/131, 118 21 Prague 1, Czech Re pub lic
2 Czech Uni ver sity of Life Sci ences Prague, Fac ulty of En vi ron men tal Sci ences, Kamýcká 129, Praha 6 – Suchdol, 165 21, Czech Re pub lic
3 Acad emy of Sci ences of the Czech Re pub lic, In sti tute of Ge ol ogy, Rozvojová 135, Praha 6, 165 00, Czech Re pub lic
Pašava, J., Frýda, J., Štorch, P., 2017. Trace el e ment vari a tions as a proxy for re con struc tion of palaeoenvironmental changes dur ing the Late Aeronian fau nal and car bon iso tope per tur ba tions: new data from the peri-Gondwanan re gion. Geo - log i cal Quar terly, 61 (1): 91–98, doi: 10.7306/gq.1313
Trace el e ment vari a tions in the Up per Aeronian (Llandovery, Lower Si lu rian), deep-wa ter, black shale suc ces sion of the Barrandian area (Perunica) were stud ied across an in ter val as so ci ated with a graptolite mass ex tinc tion and global, pos i tive car bon iso tope anom aly. The main aim of the pa per is to test whether dis tinct changes in graptolite di ver sity dur ing Late Aeronian were linked with changes in deep sea wa ter ox y gen a tion. Us ing mul ti ple geo chem i cal prox ies we doc u mented high-fre quency changes in ox y gen a tion of sea wa ter from sed i ments of the convolutus to linnaei (guerichi) biozones. De - tailed com par i son of graptolite di ver sity with those high-fre quency ox y gen a tion changes sug gests that the long-term and step-wise Late Aeronian graptolite cri sis was not sig nif i cantly in flu enced by changes in ox y gen level and thus it prob a bly re - sulted by an other causes. The col lapse of global car bon cy cle dur ing the Late Aeronian prob a bly only tem po rarily in creased ex tinc tion rate of the long-term graptolite cri sis and con sid er ably de creased even ness of the up per most Aeronian graptolite com mu ni ties. The Aeronian graptolite mass ex tinc tion was thus pri mar ily driven by other bi otic and/or abiotic causes.
Key words: aeronian, graptolite mass ex tinc tion, re dox-sen si tive trace el e ments, Barrandian area, Bo he mian Mas sif.
INTRODUCTION
Re dox-sen si tive trace el e ment (RSTE) con cen tra tions or ra tios are of ten used as in di ca tors of re dox con di tions in mod - ern and an cient sed i men tary sys tems (e.g., Hatch and Leventhal, 1992; Calvert and Pedersen, 1993; Jones and Man - ning, 1994; Wignall, 1994; Morford and Em er son, 1999;
Yarincik et al., 2000; Tribovillard et al., 2006). RSTE usu ally show sig nif i cant en rich ment in finely lam i nated, or ganic-rich fa - cies that orig i nated mostly un der anoxic/euxinic con di tions, whereas bioturbated, or ganic-poor fa cies usu ally show no metal en rich ment (e.g., Pašava et al., 1996; Pašava, 2000;
Algeo and Maynard, 2004; Tribovillard et al., 2006). Hamma - rlund et al. (2012) used geo chem i cal re dox in di ca tors, such as iron speciation, mo lyb de num con cen tra tions, py rite framboid size dis tri bu tion and sul phur iso topes for the end-Or do vi cian ex tinc tion. They con cluded that the ex pan sion of euxinic con di -
tions dur ing the extraordinarius Biozone was gen er ated by a re - or ga ni za tion of nu tri ent cy cling dur ing sea level fall, and ar gued, that these dy nam ics in ocean chem is try played an im por tant role for the end-Or do vi cian mass ex tinc tion. Most re cently, how - ever, new driv ers for the ex tinc tions have been pro posed, in - clud ing wide spread euxinia to gether with hab i tat de struc tion caused by plate tec tonic move ments, sug gest ing that the end - -Or do vi cian mass ex tinc tions were a prod uct of the co in ci dence of a num ber of con trib ut ing fac tors (Harper et al., 2014).
Vanden broucke et al. (2015) showed that metal poi son ing may have caused ab er rant morphologies dur ing a Late Si lu rian (Pridoli) event. Mal for ma tions co in cide with a dra matic in crease of met als (Fe, Mo, Pb, Mn and As) in the fos sils and their host rocks. Porêbska and Saw³owicz (1997) found that Si/Al, Si/Al/TOC, Ga/Al2O3, V/V + Ni ra tios and TOC, Mn, Ti, Re, Mo, Cd, Cu and REE cor re late with the hab i tat re place ment of the graptolite as sem blages in black shales across the Si lu rian-De - vo nian bound ary. Bond et al. (2013) re lated ma rine ex tinc tions in off shore level bot tom com mu ni ties of Frasnian -Famenian reefs (Al berta, Can ada) to two wide spread, trans gressive, ano - xic “Kellwasser Events” that sup port an an oxia–ex tinc tion link.
The Lower Si lu rian black-shales of the Prague Synform (Perunica; see Cocks and Torsvik, 2013) of fer fa vour able con - di tions for study ing the geo chem i cal sig na tures of re dox fa cies in or ganic-rich sed i men tary rocks. Štorch and Frýda (2012) re -
* Corresponding author, e-mail: jan.pasava@geology.cz Received: April 29, 2016; accepted: July 20, 2016; first published online: September 14, 2016
cently stud ied a set of sam ples from Aeronian and low er most Telychian black shale suc ces sion tem po rarily ex posed in the high way tun nel north of Radotín near Prague (Fig. 1). They fo - cused pri mar ily on the pre cise biostratigraphic tim ing and in ter - pre ta tion of the Late Aeronian graptolite mass ex tinc tion (re - ferred to as the sedgwickii Event) and the as so ci ated pos i tive car bon iso tope ex cur sion and fa cies changes. Unique, fresh ma te rial from the Radotín tun nel sec tion and its high-res o lu tion graptolite biostratigraphy pro vided a good op por tu nity for de - tailed geo chem i cal in ves ti ga tions of large, high-fre quency changes in Aeronian sea wa ter chem is try. The main aim of the pres ent pa per is to test the hy poth e sis that dis tinct changes in graptolite fau nal dy nam ics dur ing the Mid dle–Late Aeronian, cul mi nat ing in the Late Aeronian graptolite mass ex tinc tion, are linked with changes in Aeronian deep sea wa ter ox y gen a tion.
GEOLOGICAL, LITHOLOGICAL AND BIOSTRATIGRAPHICAL DESCRIPTION
Anoxic depositional set tings in the Prague Ba sin com - menced at the base of the Si lu rian Sys tem in re sponse to the Late Hirnantian-Rhuddanian anoxic event (e.g., Melchin et al., 2013) and pro duced or ganic rich shales with TOC over 2–3 wt.%, which were de pos ited un til the Early Lud low in much of the pre served out crop area of the Prague Synform (see for de tails Køíž, 1998). The com bined thick ness of the pre dom i - nantly black shale suc ces sion of the Želkovice (Rhudda - nian–Aero nian), Litohlavy (Lower and Mid dle Telychian) and Motol (Up per Telychian–Homerian) for ma tions var ies from 125 m (purely black shale suc ces sion) to 350 m (in vol ca - nic-car bon ate fa cies; Køíž, 1998). Depositional set tings, bi otic and en vi ron men tal changes, and stra tig ra phy have been stud - ied in the Lower Si lu rian suc ces sion of the Prague Synform by Bouèek (1953), Køíž (1975, 1992), Štorch and Pašava (1989), Štorch (2006), Štorch and Frýda (2012), Frýda and Štorch (2014) and oth ers. High-res o lu tion cor re la tion of lithological changes re corded in the Lower Si lu rian suc ces sion with en vi - ron men tally driven, world-wide changes is en abled by highly
de vel oped graptolite biostratigraphy (Bouèek, 1953; Štorch, 1994, 2006) and car bon iso tope geo chem is try (Frýda and Frýdová, 2014 2016; Frýda and Štorch, 2014; Frýda et al., 2014). The geo chem is try of Lower Si lu rian se quences in the Prague Synform was first stud ied by Marešová (1973), fol lowed by Lepka et al. (1984). Later, Štorch and Pašava (1989) re - corded sig nif i cant en rich ment in Mo and to a lesser de gree in Cu, Ni, Cr and S in the Lower Si lu rian black shales of the Prague Synform and sug gested that the dis tri bu tions of Mo, Cu, Ni and Pb are closely re lated to TOC.
The gen eral ge ol ogy of the Želkovice (Rhudda nian–Aero - nian) For ma tion, its graptolite biostratigraphy and car bon iso - tope chemostratigraphy, as well as de tailed de scrip tion of the Radotín tun nel sec tion (Figs. 1 and 2), were pro vided by Štorch and Frýda (2012) and Frýda and Štorch (2014). The first sig nif i cant change in the sed i men tary suc ces sion was found near the top of convolutus Biozone in the Radotín tun nel sec tion. It is as so ci ated with a grad ual but con sid er able im pov - er ish ment of high-di ver sity graptolite as sem blage. Si li ceous black shales of the convolutus Biozone pass up into py - rite-rich, graptolite-poor shale with low TOC (ca. 3 wt.%). Un - com mon graptolites re fer this shale to the lower part of sedgwickii Biozone. It is over lain by a heavily bioturbated, micaceous siltstone with out iden ti fi able graptolites (the marker bed which is char ac ter ized by pos i tive d13C anom aly), which, in turn, is suc ceeded by or ganic rich (TOC ca. 6 wt.%), finely lam i nated, micaceous black shales with an abun dant, but still lower di ver sity, graptolite fauna (Fig. 2). Fau nal dif fer ences be tween the graptolite as sem blage be low and above the siltstone (Fig. 2) and the abrupt change in grain size and TOC sug gest that an in ter val of non-de po si tion and/or ero sion may be as so ci ated with the de po si tion of the siltstone bed. The or - ganic-rich, micaceous black shales above the siltstone bed are as signed to the up per part of the sedgwickii Biozone (see Fig. 2). The strati graphi cal ranges of nu mer ous graptolite spe - cies span the in ter val. Hence the po ten tial strati graphi cal break likely rep re sents a short du ra tion, still within the mid dle part of the sedgwickii Biozone. Or ganic-rich, micaceous shales of the up per sedgwickii Biozone are over lain by black, clayey shales with abun dant graptolites of rastrum Biozone
Fig. 1. Dis tri bu tion of Si lu rian rocks in the Barrandian area, in clud ing the lo ca tions of the stud ied sec tion
im me di ately be low a thick bed of pale-col oured mudstone (Fig. 2). The mas sive bed of pale-col oured, mot tled car bon ate mudstone, with out graptolites, forms the base of the Litohlavy Fm. Low er most Telychian graptolites of linnaei Biozone (ap - prox i mate equiv a lent of guerichi Biozone) ap pear in the black claystone in ter ca la tions just above the basal mudstone. The highly di ver si fied Aeronian graptolite fauna col lected from the Radotín tun nel sec tion en abled a de tailed anal y sis of the evo - lu tion of graptolite di ver sity. Štorch and Frýda (2012) noted a close link be tween graptolite fau nal dy nam ics and car bon iso - tope anom aly in the Late Aeronian in ter val.
METHODS
In this pa per we stud ied 19 sam ples from pre dom i nantly black shale sed i men tary suc ces sion ex posed in the high way tun - nel (Fig. 1) north of Radotín, near Prague, which was pre vi ously doc u mented by Štorch and Frýda (2012). The unweathe red Llandovery black-shale suc ces sion, rich in well-pre served graptolites, yielded ex cel lent sam ples for TOC and car bon iso - tope anal y ses (Štorch and Frýda, 2012; Frýda and Štorch, 2014) as well as for anal y ses of other el e ments, in clud ing re dox sen si - Fig. 2. Strati graphi cal dis tri bu tion of d13C and TOC com po si tion and val ues of se lected ra tios
of re dox-sen si tive trace el e ments
For de tailed de scrip tion of li thol ogy see Štorch and Frýda (2012);
grey ar eas rep re sent in ter vals in ter preted to rep re sent low ox y gen level in bot tom sea wa ter
tive trace el e ments (RSTE) and rare earth el e ments (REE). As a re sult, this subsurface sec tion is of par tic u lar sig nif i cance.
The sam ples were pow dered and ana lysed for se lected ma - jor and trace el e ments in labs of the Czech Geo log i cal Sur vey.
REE, Cr, Co, Fe, Ti, Mo, Ni, Sc, Th, U, V and Zr were de ter mined us ing ICP-X Se ries. To tal sul phur was mea sured us ing the spec - tro met ric IR (Eltra) method. The TOC and d13C data are from Štorch and Frýda (2012), and Frýda and Štorch (2014).
Ce rium (Ce/Ce*) and eu ro pium (Eu/Eu*) anom a lies were cal cu lated from: Ce/Ce* = Cen/(Lan × Prn)1/2 and Eu/Eu* = Eun/(Smn × Gdn)1/2 (Tay lor and McLennan, 1985), us ing shale - -nor mal ized abun dances. Nor mal iza tion val ues are a mean of North Amer i can, Eu ro pean and Rus sian shale composites.
In this pa per, we re port whole-rock trace el e ment con cen - tra tions (Ap pen dix 1*) and es ti mate the pro por tion of each trace el e ment res i dent in the de tri tal frac tion of black shale sam ples us ing nor mal iza tion by Zr and Ti which are chiefly of de tri tal or i - gin (e.g., Calvert and Pedersen, 1993; Wignall, 1994; Algeo and Maynard, 2004 and oth ers). We show that plot ted con cen -
tra tions and val ues of metal ra tios of re dox-sen si tive trace el e - ments dom i nantly re flect their authigenic val ues and dis cuss some ex cep tions re lated to their de tri tal con tri bu tion in the fol - low ing chap ters.
RESULTS
The strati graphic dis tri bu tion of most ra tios of se lected trace el e ments is shown in Fig ures 2 and 3.
Convolutus Biozone. The d13Corg and TOC val ues of four sam ples from the convolutus Biozone are rel a tively con stant.
On the other hand, con cen tra tions of Mo and V, as well as V/Cr, V/Sc, Ce/Ce* and Eu/Eu* ra tios are vari able. The val ues of V/Cr, V/Sc, V/(V + Ni), Ce/Ce* and Eu/Eu* ra tios are high and some of those [i.e., V/Cr (9.3), V/(V + Ni) (0.9) and Eu/Eu* (2.2)]
reach their high est val ues in the stud ied sec tion (Fig. 2). In con - trast, the Th/U ra tios and Fe and S con cen tra tions reach their low est val ues (Figs. 2 and 3).
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1313
Fig. 3. Plots il lus trat ing re la tion ship of Fe and S con tents (A), Eu/Eu* and Th/U ra tios (B) V/(V + Ni) ra tio and Mo con tent (C), and V/Sc and Th/U ra tios (D)
Sedgwickii Biozone. Rel a tively low V/Cr, V/Sc, V/(V + Ni), Ce/Ce* and Eu/Eu* ra tios and Mo and V con cen tra tions and in - creased Th/U ra tios and Fe and S con cen tra tions are char ac - ter is tic fea tures of the eight sam ples from the lower part of the sedgwickii Biozone. The Th/U ra tio reaches its max i mum value (2.1) and the Mo con cen tra tion its min i mum value (0.8 ppm) in the same level, about 0.5 m be low the peak of the Late Aeronian pos i tive car bon iso tope anom aly (Fig. 2), which cor re - sponds to the top of the lower part of the sedgwickii Biozone.
The car bon iso tope anom aly is as so ci ated with very low TOC and Ni con cen tra tions (Ap pen dix 1).
Anal y ses of four sam ples from the up per part of the sedgwickii Biozone re vealed a rapid in crease in the V/Cr, V/Sc, V/(V + Ni), and Ce/Ce* ra tios and Mo, Ni, V and TOC con cen - tra tions (Ap pen dix 1). The V/Sc ra tio (61.7), as well as TOC (6.6 wt.%), Mo (39 ppm), and V (1289 ppm) con cen tra tions, reach their max i mum val ues in this in ter val within the stud ied sec tion. A no ta ble de crease of Th/U ra tios within the up per part of the sedgwickii Biozone rep re sents a cul mi na tion of a dis tinct trend, which started be low the Up per Aeronian car bon iso tope anom aly (Fig. 2).
Rastrum and linnaei biozones. Up ward de creases in the val ues of the V/Sc ra tios and TOC, Mo, and V con cen tra tions (Ap pen dix 1), as well as an in creas ing trend in the Th/U ra tio, char ac ter ize the three sam ples from the rastrum and linnaei biozones.
ENVIRONMENTAL CHANGES INFERRED FROM GEOCHEMICAL PROXIES
In the fol low ing sec tion changes in the ox y gen level in the Aeronian ma rine en vi ron ment are es ti mated based on sev eral geo chem i cal prox ies, which are dis cussed separately.
VANADIUM [V/Sc, V/Cr, V/(V + Ni)] SYSTEMATICS
Va na dium is a re dox-sen si tive el e ment that is pref er en tially con cen trated in ma rine sed i ments un der ly ing anoxic or near - -anoxic wa ters (e.g., Em er son and Huested, 1991; Rimmer, 2004). The de gree of V en rich ment is most ef fi ciently ex - pressed if the V con cen tra tion is nor mal ized by scan dium abun - dance, be cause both V and Sc are in sol u ble and V var ies in pro por tion to Sc, rather than to other in sol u ble el e ments such as Al and Ti (Kimura and Watanabe, 2001). Kimura and Wata - nabe (2001) sug gested that en vi ron ments char ac ter ized by anoxic con di tions re sulted in sig nif i cant V en rich ment over Sc.
In ad di tion, Jones and Man ning (1994) pro posed that V/Cr ra - tios <2 pointed to oxic con di tions, 2–4.25 to dysoxic con di tions, and >4.25 to suboxic to anoxic con di tions. Sim i larly, Hatch and Leventhal (1992) re ported V/(V + Ni) ra tios 0.46–0.60 for dysoxic con di tions, 0.54–0.82 for anoxic con di tions, and >0.84 for euxinic con di tions. Nor mal iza tion of V and Ni val ues by Zr showed that Ni is authigenic while V showed a slight detritic in - put in the up per part of the sedgwickii Biozone and also in rastrum and linnaei Biozone. In this con text, the four sam ples from the convolutus Biozone with high val ues of the V/Sc ra tio (17.3–49.7; avg. 28.8), V/Cr ra tio (2.8–9.3, avg. 5.2) and V/(V + Ni) ra tio (0.7–0.9; avg. 0.8; Fig. 3) clearly in di cate dysoxic to anoxic bot tom-wa ter con di tions (Fig. 3). This con clu sion fits also with the high est en rich ment in Mo (21.5–43.9 ppm; avg.
30 ppm), but does not, how ever, co in cide with the high est TOC val ues (2.65–4.09 wt.%; avg. 3.25 wt.%). A dif fer ent sit u a tion can be ob served in the sedgwickii Biozone. In the lower part all
eight sam ples show much lower ra tios of V/Sc (5.5–11.7; avg.
7.7), V/Cr (0.7–1.8; avg. 1.1), and V/(V + Ni) (0.4–0.7; avg. 0.6;
Fig. 3) in di cat ing oxic to dysoxic con di tions (Fig. 4). A lack of sig nif i cant cor re la tion be tween re dox sen si tive el e ments and TOC was prob a bly in flu enced by in creas ing terrigenous in flux just be fore the end of the time in ter val of the lower part of the sedgwickii Biozone (Fig. 3). Four sam ples from the up per part of the sedgwickii Biozone show a re turn to much higher V/Sc ra - tios (33.7–61.7; avg. 45.3) and also higher val ues of V/Cr (2.4–3.4; avg. 3.1) and V/V + Ni (0.77–0.84; avg. 0.81; Fig. 3).
The lat ter high val ues, to gether with higher con cen tra tions of Mo (19.2–38.6 ppm; avg. 27.5 ppm) and TOC (5.7–6.6 wt.%;
avg. 6.2 wt.%), clearly in di cate re turn to dyso xic/anoxic deep - -wa ter en vi ron ment. Three sam ples from the rastrum–linnaei biozones re vealed an up ward de creas ing trend in val ues of V/Sc (from 40.0 to 17.2) and V/Cr (from 3.6 to 1.0) ra tios, and low val ues of V/V + Ni (0.5–0.7; Fig. 2), doc u ment ing a re turn to oxic con di tions. This con clu sion is sup ported by low Mo (2.5–2.8 ppm) and TOC val ues (2.68–2.80 wt.%).
Th/U RATIO
The Th/U ra tio has been used as a proxy for the re dox con - di tion of the depositional en vi ron ment (e.g., Myers and Wignall, 1987; Wignall, 1994). Tho rium is un af fected by re dox con di - tions and re mains in sol u ble as Th4+. Ura nium, how ever, ex ists as in sol u ble U4+ un der highly re duc ing con di tions, which leads to U en rich ment in sed i ments, whereas it ex ists as sol u ble U6+
un der ox i diz ing con di tions, lead ing to the U loss from sed i ments (Kimura and Watanabe, 2001). Wignall and Twitchett (1996) sug gested that en vi ron ments char ac ter ized by anoxic con di - tions yielded Th/U ra tios of 0 to 2 in shales, and this cri te rion has been widely ac cepted in sub se quent stud ies (e.g., Kimura and Watanabe, 2001; Guo et al., 2007).
The four stud ied sam ples from the convolutus Biozone have very low val ues of the Th/U ra tios (0.11–0.29; avg. 0.21), which is typ i cal for anoxic en vi ron ments. Nev er the less, nor mal - iza tion of U val ues by Zr in di cates a pos si ble slight detritic in put of U in the up per part of the convolutus Biozone sim i larly as V.
On the other hand, val ues of the Th/U ra tios are in creas ing from the base of the lower part of the sedgwickii Biozone to its mid dle where the Th/U ra tio reaches the max i mum value of 2.1, sug - gest ing more oxic con di tions rel a tive to older strata. Sub se - quently val ues of the Th/U ra tios grad u ally de crease (Fig. 2) from 2.1 to 0.4 at the base of the up per part of the sedgwickii Biozone. These val ues clearly in di cate a tran si tion back to anoxic con di tions, sim i lar to those in the convolutus Biozone (Figs. 2 and 3). Anoxic con di tions re mained steady dur ing the time of the up per part of the sedgwickii Biozone. Above the top of the sedgwickii Biozone the Th/U val ues again show a con sid - er able in crease in di cat ing a dis tinct change from strongly anoxic to oxic en vi ron ments (Figs. 3 and 4).
Fe/S RATIO
The Fe/S ra tio can be used as an in di ca tor for the amount of sul phide-bound Fe in sed i men tary strata – a lower value of the Fe/S ra tio in di cates that more of the Fe is sul phide-bound (e.g., März et al., 2009). The Fe/S ra tio in the stud ied sam ples ranges be tween 0.9 and 6.2. The vast ma jor ity of sam ples are close to the “py rite line” but Fe-en riched (Fig. 3), sug gest ing that py rite is the main phase con trol ling the con tent of Fe and S. The max i - mum value of the Fe/S ra tio was mea sured in black shale of the linnaei Biozone (Fig. 3) and co in cides with the low est S/TOC ra -
tio (0.3), a low value of V/Cr and high Th/U (1.6) and also the high est Th value (Ap pen dix 1). This could in di cate a terri - genous/volcanogenic in put in clud ing de tri tal Fe (oxyhydr)ox - ides or other Fe-bear ing min er als in a more oxic en vi ron ment.
Ce/Ce* AND Eu/Eu* RATIOS
Nor mal sea wa ter shows a marked de ple tion of Ce, ow ing to the pre dom i nance of ox i da tion to Ce4+ and ac cu mu la tion in the sed i ments as CeO2. In con trast, at a lo cal re dox-cline such as that in the Black Sea, Ce is re duced to Ce3+ and the Ce anom aly is not pres ent (Ras mus sen et al., 1998). Wright et al. (1987) and Wignall (1994) sug gested that the Ce anom aly can re cord the re - dox con di tions of the over ly ing sea wa ter col umn, and that this sig na ture re mains in spite of sub se quent burial and later dia - genesis. Eu/Eu* anom a lies have been re ported in many types of ig ne ous and sed i men tary rocks and Eu anom a lies play a sig nif i - cant role in in ter pret ing the phys i cal and chem i cal con di tions (e.g., Derry and Jacobsen, 1990; MacRae et al., 1992; Fedo et al., 1996; Ras mus sen et al., 1998; Lee et al., 2004; Sugitani et al., 2006 and oth ers). Nor mal iza tion of Ce and Eu val ues by Zr showed that both el e ments could in di cate a lo cal detritic in put – Ce in the up per part of the sedgwickii Biozone in clud ing the Up - per Aeronian car bon iso tope anom aly and also in the ras trum and linnaei biozones while Eu only close to the Up per Aeronian car bon iso tope anom aly. Dom i nant terrigenous sour ce of REE is in di cated by the val ues of Y/Ho ra tio which range from 28 to 38 (me dian value 31) as Y/Ho ra tio of about 28 is typ i cal for com - mon ig ne ous rocks and clastic sed i ments (Bau, 1996).
The cal cu lated Ce/Ce* val ues in the stud ied sam ples vary in a nar row range, from 0.73 to 1.02. The max i mum value of the Ce/Ce* ra tio (1.02) co mes from the heavily bioturbated mica - ceous siltstone de pos ited dur ing the Late Aeronian car bon iso - tope anom aly. The rest of the sam ples have av er age val ues of Ce/Ce* ra tios (0.85) which fits well with a range of black shales de pos ited un der low ox y gen con di tions (e.g., Yan et al., 2009).
On the other hand, the Eu/Eu* val ues of the Radotín tun nel
sam ples are high rang ing from 1.43 to 2.19 (avg. 1.62). The high val ues of the Eu/Eu* ra tio (in clud ing the max i mum value of 2.19) oc cur in the convolutus Biozone (Fig. 2) and in the up per part of the sedgwickii Biozone, sug gest ing an ox y gen-de fi cient en vi ron ment. A neg a tive cor re la tion be tween the Eu/Eu* and Th/U val ues in di cates a de pend ence of the mag ni tude of the neg a tive Eu* anom aly on the de gree of anoxicity (Fig. 3).
Taken to gether, the dif fer ent geo chem i cal prox ies that we have mea sured sug gest al most a re cur ring sce nario of the deep-wa ter ox y gen level changes dur ing the stud ied time span, from the convolutus to linneai biozones (Fig. 4). Dis tinctly anoxic to dysoxic con di tions pre vailed dur ing time of the convo - lutus Biozone. From the be gin ning of the sedgwickii Biozone time in ter val the ox y gen level in deep sea wa ter was con sid er - ably higher, chang ing to dysoxic/oxic con di tions in the mid dle of the lower part of the sedgwickii Biozone [max i mum value of the Th/U, min i mum value of V/(V + Ni) and Mo, etc.]. A pro nounced re duc tion in ox y gen level oc curred just above the Late Aeronian car bon iso tope anom aly (Fig. 4). The geo chem i cal prox ies em - ployed here sug gest a rapid shift to a more dysoxic/anoxic en vi - ron ment, which was sta ble un til the end of the sedgwickii Biozone. The pe riod of the rastrum–linnaei biozones is char ac - te r ized by a re turn to oxic con di tions (Fig. 4).
RELATIONSHIP OF GRAPTOLITE FAUNAL DYNAMICS AND ENVIRONMENTAL CHANGES
Štorch and Frýda (2012) ana lysed graptolite fau nal dy - nam ics dur ing the Mid dle–Late Aeronian, from the leptotheca to rastrum biozones, based on the data from the Radotín tun - nel sec tion as well as all other avail able data from the Barrandian area. The anal y ses of data from the Radotín tun nel sec tion, us ing dif fer ent di ver sity in di ces, re vealed a sta tis ti - cally sig nif i cant de creas ing trend in graptolite spe cies di ver sity from the leptotheca to lower sedgwickii biozones and a fur ther, dis tinct de crease in grap tolite spe cies rich ness be tween the lower and up per parts of the sedgwickii Biozone (Fig. 4; Štorch
Fig. 4. Di a gram show ing d13C re cord as well as spe cies rich ness and dom i nance in dex cal cu lated for in di vid ual sam ples from the Radotín tun nel sec tion
For li thol ogy de scrip tion see Fig ure 2; grey ar eas rep re sent in ter vals in ter preted to rep re sent low ox y gen level in bot tom sea wa ter
and Frýda, 2012: fig. 7a, b). On the other hand, no sta tis ti cally sig nif i cant trend in spe cies rich ness was found for data from the up per sedgwickii and rastrum biozones. These re sults are con sis tent with the gen eral trend in the num ber of graptolite spe cies re corded from leptotheca to rastrum biozones at all avail able sec tions of the Barrandian area (see Štorch and Frýda, 2012: fig. 7d).
The anal y sis of even ness of graptolite com mu ni ties re - vealed a sta tis ti cally sig nif i cant trend in in creas ing dom i nance within graptolite as sem blages across the stud ied in ter val (Fig.
4). A more de tailed view, how ever, shows that the dom i nance in dex (see Ham mer and Harper, 2008) has quite dif fer ent range for data from the leptotheca to lower sedgwickii biozones on the one hand and from data from the up per sedgwickii and rastrum biozones on the other (Fig. 4). In the first strati graphi cal in ter val the dom i nance var ies only slightly but it in creased dra mat i cally (three times) in three sam ples of the up per sedgwickii and low - er most rastrum biozones (Fig. 4).
Štorch and Frýda (2012) showed that a quan ti ta tive study of graptolite fau nal dy nam ics has pro vided ev i dence for the com - plex and step-wise na ture of the Late Aeronian graptolite cri sis, which ex tended through the whole sedgwickii Biozone and well into the rastrum Biozone. They also noted that the rapid in - crease in the ex tinc tion rate of graptolite spe cies started near the end of the convolutus Biozone and ini tially was partly bal - anced by an in creased rate of graptolite speciation and/or im mi - gra tion. The most dis tinc tive de crease of the graptolite spe cies di ver sity oc curs in the mid dle of the sedgwickii Biozone, co in ci - dent with the Late Aeronian car bon iso tope anom aly, which is as so ci ated with ma jor sea-level drawdown.
Our new data sug gest that the long-term and step-wise Late Aeronian graptolite cri sis, which started near the end of the leptotheca Biozone, was most likely not strongly in flu enced by changes in ox y gen level. There is no cor re la tion be tween the spe cies rich ness of graptolite com mu ni ties and the level of sea wa ter ox y gen a tion in ferred from geo chem i cal prox ies (Fig. 4).
We doc u ment the in ter vals with sim i lar en vi ron men tal con di - tions have quite dif fer ent graptolite spe cies rich ness (see Fig. 4;
com pare a di ver sity of anoxic to dysoxic convolutus and up per sedgwickii biozones, or that of dysoxic to oxic lower sedgwickii, rastrum and linnaei biozones). On the other hand, the up per sedgwickii and rastrum biozones have sim i lar (but both highly vari able) lev els of graptolite spe cies rich ness re gard less dif fer - ences in ox y gen level.
Col lapse of the global car bon cy cle dur ing the Late Aero - nian anom aly only em pha sized the long-term and step-wise na -
ture of the graptolite cri sis but it con sid er ably de creased even - ness of the lat est Aeronian graptolite com mu ni ties. Only lo cally there seem to be a link be tween ox y gen a tion and short-term, abrupt changes in graptolite di ver sity and dominance.
CONCLUSIONS
The main goal of this study was to test the hy poth e sis that dis tinct changes in graptolite fau nal dy nam ics dur ing the Aeronian, as so ci ated with the Late Aeronian graptolite mass ex tinc tion, are linked with changes in deep sea wa ter ox y gen a - tion. Our data show that anoxic to dysoxic con di tions pre vailed dur ing the in ter val cor re spond ing to the convolutus Biozone.
From the be gin ning of the sedgwickii Biozone in ter val ox y gen level in deep sea wa ter was con sid er ably high, cre at ing dysoxic to oxic con di tions shortly be fore the time of the Late Aeronian car bon iso tope pos i tive anom aly. A rapid shift to a more dyso - xic/anoxic en vi ron ment oc curred just af ter time of this anom aly and these con di tions re mained steady un til the end of the sedgwickii Biozone in ter val. The ox y gen level again in crea sed dur ing the time of the rastrum and linnaei biozones.
Com par i son of graptolite di ver sity with these high-fre quen - cy changes in ox y gen a tion of deep sea wa ter re vealed that the long-term and step-wise Late Aeronian graptolite cri sis was most likely not sig nif i cantly in flu enced by changes in ox y gen level ex cept at the time of the C-iso tope ex cur sion. The col lapse of global car bon cy cle dur ing the Late Aeronian prob a bly only tem po rarily in creased ex tinc tion rate of the long-term graptolite cri sis and con sid er ably de creased even ness of the up per most Aeronian graptolite com mu ni ties. The long-term Aeronian grapto lite mass ex tinc tion was prob a bly mainly driven by other biotic and/or abiotic causes.
Ac knowl edge ments. This pa per is a con tri bu tion to IGCP 591 and IGCP 596. The re search was sup ported by a grant from the Czech Geo log i cal Sur vey (338800) and two grants from the Grant Agency of the Czech Re pub lic (GAÈR 13-15390 S and GAÈR 15-13310 S). We are very grate ful to M.J. Melchin for help ful com ments on the manu script and the Eng lish cor rec - tion. The manu script has bene fited from stim u lat ing re views by S. Oszczepalski, Z. Saw³owicz and one anon y mous re viewer.
Ef fi cient ed i to rial han dling by T. Peryt and E. D¹browska - -Jêdrusik is greatly appreciated.
REFERENCES
Algeo, T.J., Maynard, J.B., 2004. Trace el e ment be hav iour and re - dox fa cies in core shales of the Up per Penn syl va nian Kan - sas-type cyclothems. Chem i cal Ge ol ogy, 206: 289–318.
Bau, M., 1996. Con trols on the frac tion ation of isovalent trace el e - ments in mag matic and aque ous sys tems: ev i dence from Y/Ho, Zr/Hf, and lanthanide tetrad ef fect. Con tri bu tions to Min er al ogy and Pe trol ogy, 123: 323–333.
Bond, D.P.G., Zato, M., Wignall, P.B., Marynowski, L., 2013. Ev i - dence for shal low wa ter “Up per Kellwasser” an oxia in the Fra - snian–Fa mennian reefs of Al berta, Can ada. Lethaia, 46: 355–368.
Bouèek, B., 1953. Biostratigraphy, de vel op ment and cor re la tion of the Želkovice and Motol Beds of the Si lu rian of Bo he mia. Sbor - ník Ústøedního Ústavu Geologického, Oddíl Paleontologický, 20: 421–484.
Calvert, S.E., Pedersen, T.F., 1993. Geo chem is try of re cent oxic and anoxic ma rine sed i ments: im pli ca tions for the geo log i cal re - cord. Ma rine Ge ol ogy, 113: 67–88.
Cocks, L.R.M., Torsvik, T.H., 2013. New global palaeo geo graphi - cal re con struc tions for the Early Palaeozoic and their gen er a - tion. Geo log i cal So ci ety of Lon don Mem oirs, 38: 5–24.
Derry, L.A., Jacobsen, S.B., 1990. The chem i cal evo lu tion of Pre - cam brian sea wa ter: ev i dence from REEs in banded iron for ma - tions. Geochimica et Cosmochimica Acta, 49: 1955–1963.
Em er son, S.R., Huested, S.S., 1991. Ocean an oxia and the con - cen tra tions of mo lyb de num and va na dium in sea wa ter. Ma rine Ge ol ogy, 34: 177–196.
Fedo, C.M., Eriksson, K.A., Krogstad, E.J., 1996. Geo chem is try of shales from the Archean (~3.0 Ga) Buhwa Greenstone Belt,
Zim ba bwe: im pli ca tions for prov e nance and source-area weath - er ing. Geochimica et Cosmochimica Acta, 60: 1751–1763.
Frýda, J., Frýdová, B., 2014. First ev i dence for the Homerian (late Wen lock, Si lu rian) pos i tive car bon iso tope ex cur sion from peri-Gond wana: new data from the Barrandian (Perunica). Bul - le tin of Geosciences, 89: 617–634.
Frýda, J., Frýdová, B., 2016. The Homerian (late Wen lock, Si lu - rian) car bon iso tope ex cur sion from Perunica: does do lo mite con trol the mag ni tude of the car bon iso tope ex cur sion? Ca na - dian Jour nal of Earth Sci ences, 53: 695–701.
Frýda, J., Štorch, P., 2014. Car bon iso tope chemostratigraphy of the Llandovery in north ern peri-Gond wana: new data from the Barrandian area, Czech Re pub lic. Es to nian Jour nal of Earth Sci ences, 63: 220–226.
Frýda, J., Lehnert, O., Joachimski, M.M., 2014. First re cord of the early Sheinwoodian car bon iso tope ex cur sion (ESCIE) from the Barrandian area of north west ern peri-Gond wana. Es to nian Jour nal of Earth Sci ences, 64: 42–46.
Guo, Q., Shields, G.A., Liu, C., Strauss, H., Zhu, M., Pi, D., Goldberg, T., 2007. Trace el e ment chemostratigraphy of two Ediacaran-Cam brian suc ces sions in South China: im pli ca tions for organosedimentary metal en rich ment and sili ci fi ca tion in the early Cam brian. Palaeo ge ogra phy, Palaeoclimatology, Palaeo - ec ol ogy, 254: 194–216.
Hammarlund, E.U., Dahl, T.W., Harper, D.A.T., Bond, D.P.G., Niel - sen, A.T., Bjerrum, C.J., Schovsbo, N.H., Schönlaub, H.P., Zalasiewicz, J.A., Can field, D.E., 2012. A sulfidic driver for the end-Or do vi cian mass ex tinc tion. Earth and Plan e tary Sci ence Let ters, 331–332: 128–139.
Ham mer, Ø., Harper, D., 2008. Paleontological Data Anal y sis.
Willey-Blackwell, Ox ford.
Harper, D.A.T., Hammarlund, E.U., Ras mus sen, C.M.Ø., 2014.
End Or do vi cian ex tinc tions: a co in ci dence of causes. Gond - wana Re search, 25: 1294–1307.
Hatch, J.R., Leventhal, J.S., 1992. Re la tion ship be tween in ferred re dox po ten tial of the depositional en vi ron ment and geo chem is - try of the Up per Penn syl va nian (Mis sou rian) stark shale mem - ber of the Den nis Lime-stone, Wabaunsee County, Kan sas, USA. Chem i cal Ge ol ogy, 99: 65–82.
Jones, B., Man ning, D.A.C., 1994. Com par i son of geo chem i cal in - di ces used for the in ter pre ta tion of palaeoredox con di tions in an - cient mudstones. Chem i cal Ge ol ogy, 111: 111–129.
Kimura, H., Watanabe, Y., 2001. Ocean an oxia at the Pre cam - brian-Cam brian bound ary. Ge ol ogy, 29: 995–998.
Køíž, J., 1975. Re vi sion of the Lower Si lu rian stra tig ra phy in Cen tral Bo he mia. Vìstník Ústøedního Ústavu Geologického, 50: 275–282.
Køíž, J., 1992. Si lu rian field ex cur sions: Prague Ba sin (Barrandian), Bo he mia. Na tional Mu seum Wales, Geo log i cal Se ries, 13: 1–111.
Køíž, J., 1998. Si lu rian. In: Pa leo zoic of the Barrandian (Cam brian to De vo nian) (eds. I. Chlupáè, V. Havlíèek, J. Køíž, Kukal and P.
Štorch): 79–101. Èeský Geologický Ústav, Praha.
Lee, S.G., Kim, Y., Chae, B.G., Koh, D.C., Kim, K.H., 2004. The geo chem i cal im pli ca tion of a vari able Eu anom aly in a frac tured gneiss core: ap pli ca tion for un der stand ing Am be hav ior in the geo log i cal en vi ron ment. Ap plied Geo chem is try, 19: 1711–1725.
Lepka, F., Podracký, P., Knotek, M., 1984. Pos si bil i ties of the use of geo chem i cal cri te ria for strati graphic di vi sion of silty-clayey sed i ments of Barrandian (in Czech). Acta Montana, 67: 59–74.
MacRae, N.D., Nesbitt, H.W., Kronberg, B.I., 1992. De vel op ment of a pos i tive Eu anom aly dur ing diagenesis. Earth and Plan e tary Sci ence Let ters, 109: 585–591.
Marešová, Z., 1973. Geo log i cal set ting and pos si bil i ties of ura nium dis tri bu tion in the area of Barrandian lower Palaeozoic (in Czech). Open File Re port, Ar chive of the Czecho slo vak Ura - nium In dus try, Pøíbram.
März, Ch., Beck mann, B., Franke, Ch., Vogt, Ch., Wag ner, T., Kasten, S., 2009. Geo chem i cal en vi ron ment of the Coniacian - -Santonian west ern trop i cal At lan tic at Dem er ara Rise. Palaeo - ge ogra phy, Palaeoclimatology, Palaeo ec ol ogy, 273: 286–301.
Melchin, M.J., Mitch ell, C.E., Holmden, C., Štorch, P., 2013. En vi - ron men tal changes in the Late Or do vi cian–early Si lu rian: re view and new in sights from black shales and ni tro gen iso topes. GSA Bul le tin, 125: 1635–1670.
Morford, J.L., Em er son, S., 1999. The geo chem is try of re dox sen - si tive trace met als in sed i ments. Geochimica et Cosmochimica Acta, 63: 1735–1750.
Myers, K.J., Wignall, P.B., 1987. Un der stand ing Ju ras sic or - ganic-rich mud-rocks – new con cepts us ing gamma ray spec - trom e try and palaeo ec ol ogy: ex am ples from the Kimmeridge clay of Dorset and the Jet rock of York shire. In: Ma rine Clastic Sedimentology (eds. J.K. Leggett and G.G. Zuffa): 175–192.
Gra ham and Trotman, Lon don.
Pašava, J., 2000. Nor mal ver sus metal-rich black shales in the Barrandian Neoproterozoic of the Teplá-Barrandian Unit: a sum - mary with new data. Vìstník Èeského Geologického Ústavu, 75:
229–239.
Pašava, J., Hladiková, J., Dobeš, P., 1996. Or i gin of Pro tero zoic metal-rich black shales from the Bo he mian Mas sif, Czech Re - pub lic. Eco nomic Ge ol ogy, 91: 63–79.
Porêbska, E., Saw³owicz, Z., 1997. Palaeoceanographic link age of geo chem i cal and graptolite events across the Si lu rian-De vo nian bound ary in Bardzkie Moun tains (South west Po land). Palaeo - ge ogra phy Palaeoclimatology Palaeo ec ol ogy, 132: 343–354.
Ras mus sen, B., Bu ick, R., Tay lor, W.R., 1998. Re moval of oce anic REE by authigenic pre cip i ta tion of phos phatic min er als. Earth and Plan e tary Sci ence Let ters, 164: 135–149.
Rimmer, S.M., 2004. Geo chem i cal paleoredox in di ca tors in De vo - nian-Mis sis sip pian black shales, cen tral Ap pa la chian Ba sin (U.S.A.). Chem i cal Ge ol ogy, 206: 373–391.
Sugitani, K., Yamashita, F., Nagaoka, T., Minami, M., Yamamoto, K., 2006. Geo chem is try of heavily al tered Archean vol ca nic and volcaniclastic rocks of the Warrawoona Group, at Mt. Golds - worthy in the Pilbara Craton, West ern Aus tra lia: im pli ca tions for al ter ation and or i gin. Geo chem i cal Jour nal, 40: 523–535.
Štorch, P., 1994. Graptolite biostratigraphy of the Lower Si lu rian (Llandovery and Wen lock) of Bo he mia. Geo log i cal Jour nal, 29:
137–165.
Štorch, P., 2006. Fa cies de vel op ment, depositional set tings and se - quence stra tig ra phy across the Or do vi cian-Si lu rian bound ary: a new per spec tive from Barrandian area of the Czech Re pub lic.
Geo log i cal Jour nal, 41: 163–192.
Štorch, P., Frýda, J., 2012. The late Aeronian graptolite sedgwickii Event, as so ci ated pos i tive car bon iso tope ex cur sion and fa cies changes in the Prague Synform (Barrandian area, Bo he mia).
Geo log i cal Mag a zine, 149: 1089–1106.
Štorch, P., Pašava, J., 1989. Stra tig ra phy, chem is try and or i gin of the Lower Si lu rian black grap to lit ic shale of the Prague Ba sin, Barrandian, Bo he mia. Vìstník Èeského Geologického Ústavu, 64: 143–162.
Tay lor, S.R., McLennan, S.M., 1985. The Con ti nen tal Crustal: its Com po si tion and Evo lu tion. Blackwell, Ox ford.
Tribovillard, N., Algeo, T.J., Ly ons, T., Riboulleau, A., 2006. Trace met als as paleoredox and paleoproductivity prox ies: an up date.
Chem i cal Ge ol ogy, 232: 12–32.
Vandenbroucke, T.R.A., Emsbo, P., Munnecke, A., Nuns, N., Duponchel, L., Lepot, K., Quijada, M., Paris, F., Servais, T., Kießling, W., 2015. Metal-in duced mal for ma tions in Early Palaeozoic plank ton are har bin gers of mass ex tinc tion. Na ture Com mu ni ca tions, 6: 7966.
Wignall, P.B., 1994. Black Shales. Ox ford Uni ver sity Press, New York.
Wignall, P.B., Twitchett, R.J., 1996. Oce anic an oxia and the end Perm ian mass ex tinc tion. Sci ence, 272: 1155–1158.
Wright, J., Schrader, H., Holser, W.T., 1987. Paleoredox vari a tions in an cient oceans re corded by rare earth el e ments in fos sil ap a - tite. Geochimica et Cosmochimica Acta, 51: 631–644.
Yan, D.T., Chen, D.Z., Wang, Q.Ch., Wang, J.G., 2009. Geo chem i - cal changes across the Or do vi cian-Si lu rian tran si tion on the Yang tze Plat form, South China. Sci ence in China, Se ries D-Earth Sci ences, 52: 38–54.
Yarincik, K.M., Murray, R.W., Ly ons, T.W., Pe ter son, L.C., Haug, G.H., 2000. Ox y gen a tion his tory of bot tom wa ters in the Cariaco Ba sin, Ven e zuela, over the past 578,000 years: re sults from re - dox-sen si tive met als (Mo, V, Mn, and Fe). Paleoceanography, 15: 593–604.
