Depositional en vi ron ments of the Car bon if er ous-Perm ian Taiyuan For ma tion (south ern North China Block) as de duced from trace el e ments
and from car bon and ox y gen iso topes
Dawei LV1, 2, Guangqing HU3, 4, A.J. (Tom) VAN LOON5, * and Dun WU3, 4
1 Shandong Uni ver sity of Sci ence and Tech nol ogy, Shandong Pro vin cial Key Lab o ra tory of Depositional Min er al iza tion and Sed i men tary Min er als, Qingdao, Shandong 266590, China
2 Lab o ra tory for Ma rine Min eral Re sources, Qingdao Na tional Lab o ra tory for Ma rine Sci ence and Tech nol ogy, Qingdao, Shandong 266237, China
3 Uni ver sity of Sci ence and Tech nol ogy of China, School of Earth and Space Sci ences, Hefei, Anhui 230026, China
4 Ex plo ra tion and Re search In sti tute, Anhui Pro vin cial Bu reau of Coal Ge ol ogy, Hefei 230088, China
5 Shandong Uni ver sity of Sci ence and Tech nol ogy, Col lege of Earth Sci ence and En gi neer ing, Qingdao, Shandong 266590, China
Lv, D., Hu, G., Van Loon A.J. (Tom), Wu, D., 2021. Depositional en vi ron ments of the Car bon if er ous-Perm ian Taiyuan For - ma tion (south ern North China Block) as de duced from trace el e ments and from car bon and ox y gen iso topes. Geo log i cal Quar terly, 2021, 65: 1, doi: 10.7306/gq.1570
Lime stone lay ers are ex ten sively de vel oped in the con ti nen tal/ocean tran si tional Taiyuan For ma tion. This for ma tion ac cu - mu lated on the south ern North China Block. The pre cise en vi ron men tal con di tions of the Taiyuan For ma tion are still con tro - ver sial. More in for ma tion about these top ics is pre sented here on the ba sis of the anal y sis of trace el e ments (Mo, V, Cd, Cr, U, Th), which can be used to de ter mine char ac ter is tics of the depositional en vi ron ment, and of car bon and ox y gen iso topes (d13C and d18O) in lime stone sam ples from the Huainan Coal Ba sin. Sam ples were taken for the pur pose from cores of all 13 lime stone lev els, ob tained from a coal-ex plo ra tion borehole (code P2) in the Huainan Coal Ba sin. It was found that the d18O val ues show a de creas ing trend, sug gest ing a grad ual rise of the sea level in a warm cli mate. Three neg a tive shifts of d13C ap pear in a lower, a mid dle and an up per lime stone layer, ac com pa nied by rel a tive en rich ment of the re dox-sen si tive el e - ments (Cd, Cr, Mo and V). These three lay ers are thus de duced to have been de pos ited in a warm cli mate with a high sea level and with more terrigenous in put than dur ing de po si tion of the other lime stone lay ers. The re dox el e ments and el e men tal ra tios (V/Cr, Th/U) in the lime stones sug gest de po si tion in an ox y gen-rich sea with high sa lin ity.
Key words: Taiyuan For ma tion, North China Block, palaeoenvironmental re con struc tion, trace-el e ment anal y sis.
INTRODUCTION
The Car bon if er ous-Perm ian tran si tion is an im por tant time-span be cause the Earth changed from a green house to an ice house, and ex pe ri enced in ten sive glacio-eustatic fluc tu a - tions (e.g., Bruckschen et al., 1999; Golonka and Ford, 2000;
Sahney et al., 2010; Montañez and Cecil, 2013). More over, the Pangea supercontinent was formed by col li sion of sev eral plates (Isbell et al., 2008). Coal-bear ing suc ces sions de vel oped in the low-lat i tude ar eas (among which the North China Block) on the con ti nents, whereas the char ac ter is tics of the oceans (e.g., Eh, sa lin ity, de tri tal in put and life forms) be came more dif -
fer en ti ated due to the global changes in palaeoclimate (Scheffler et al., 2003).
The North China Block (NCB) was dur ing the Car bon if er ous sit u ated at the east ern mar gin of the Palaeotethys Ocean, on both sides of the equa tor (Golonka and Ford, 2000; Zheng et al., 2013). At the end of the Car bon if er ous, an epicontinental sea ex isted on the NCB, in which the Taiyuan For ma tion ac cu - mu lated. This con tin ued till in the Early Perm ian (Lv and Chen, 2014). Suc ces sive trans gres sions and re gres sions that oc - curred while the for ma tion was build ing up re sulted in a suc ces - sion with al ter nat ing lime stones, coals and clastic sed i ments.
Ma rine sed i men tary suc ces sions on the NCB pre served, like such suc ces sions else where in the world, changes in the char ac ter is tics of the sea wa ter, par tic u larly its geo chem is try.
This im plies that anal y sis of these sed i ments can help re con - struct ing the oce anic de vel op ment if the right clues are found. Is has be come clear that par tic u larly the iso tope and el e ment com po si tions of ma rine car bon ates tend to re flect the orig i nal geo chem i cal sig na ture of the ocean, and that the iso tope and el e ment com po si tions are con trolled by fac tors such as the in -
* Corresponding author, e-mail: Geocom.VanLoon@gmail.com Received: July 20, 2020; accepted: October 12, 2020; first published online: December 13, 2020
put of terrigenous ma te rial, the sol u bil ity of the el e ments, and the Eh of the sea wa ter (Wignall et al., 2007). Re gard ing the first as pect, weath er ing of the con ti nen tal crust is the main pro cess re leas ing el e ments that even tu ally reach the ocean, while the sec ond as pect, i.e. the sol u bil ity of el e ments in the sea wa ter, largely de pends on the char ac ter is tics of the el e ments and the re dox po ten tial of the wa ter. In sol u ble el e ments, such as Al, Sc, Hf, Th and REE, tend to be come in cor po rated in car bon ates along with par ti cles, form ing im pure car bon ates (Frimmel, 2009; Zhao et al., 2009). In con trast, eas ily dis solved el e ments such as Na, Mg and U are in cor po rated as ions in the crys tal lat - tice of the car bon ates, form ing pure car bon ates. Anal y sis of the el e ment com po si tion of ma rine car bon ates is con se quently widely ap plied for the re con struc tion of the palaeoenvironments of the ocean.
The car bon and ox y gen iso tope anom a lies (de noted as d13C and d18O) in car bon ates also de pend on the depositional en vi ron ment. The d13C and d18O val ues are im por tant geo - chem i cal tools for study ing not only the depositional en vi ron - ment (Pufahl et al., 2006; Payne and Kump, 2007) but also the palaeoclimate (Kaufman et al., 1993; Kump and Ar thur, 1999;
Korte et al., 2005). They have also widely been used in un rav el - ing the global car bon cy cle, gla cial/inter gla cial cy cles, bi o log i cal foot prints, tem per a ture and sea level fluc tu a tions, and the evo - lu tion of the sea wa ter (Scheffler et al., 2003; Jaffrés et al., 2007; Veizer and Prokoph, 2015). The pos i tive and neg a tive ex cur sions of d13C and d18O in car bon ates are com monly linked to im por tant global events. For ex am ple, Bruckschen et al.
(1999) showed that the over all 3–4‰ in crease of d13C with d18O in Car bon if er ous brachi o pods is re lated to the tran si tion from the Tournaisian green house to the Gzhelian ice house.
OBJECTIVES OF THE STUDY
Sev eral stud ies have been de voted to the de vel op ment in pres ent-day China of the Car bon if er ous and Perm ian de vel op - ment in the Palaeotethys Ocean (e.g., Lee and Lee, 2003; Lv and Chen, 2014), of ten with a fo cus on the palae on tol ogy (Tazawa, 2002), the prov e nance of the clastics (Li et al., 2010;
Zhang et al., 2014) and the stra tig ra phy (Han, 1990; Shao et al., 2015) which is partly based on the pres ence of vol ca nic tuff lay - ers (Schmitz et al., 2020), but the depositional en vi ron ments of the epicontinental sea in which the Taiyuan For ma tion ac cu mu - lated are still a mat ter of de bate (Guo and Liu, 2000; Li and Wang, 2006; Song et al., 2011), mainly be cause in ter pre ta tions of the fos sils, which may partly have been washed in, are mu tu - ally in con sis tent (Song et al., 2015; Mao et al., 2018; Wang et al., 2019). The most im por tant groups of fos sils found in the Taiyuan For ma tion are plants, brachi o pods, gas tro pods, cor - als, cri noids, fusulinids and ostracods (Ge et al., 1985; Gao, 1988; Fan et al., 1999; Wan et al., 2017; Zhang, 2017). The or i - gin of the fos sils (more or less in place or washed in from far away) is also a point of dis cus sion, be cause some stud ies state that the lime stones were formed in an ox i diz ing en vi ron ment (e.g., Guo and Liu, 2000), whereas other stud ies men tion a re - duc ing en vi ron ment (e.g., Song et al., 2011).
The ob jec tive of the pres ent con tri bu tion is there fore to shed more light on the depositional con di tions. For the pur pose, the char ac ter is tics of the sea wa ter were in ves ti gated by means of anal y ses of the iso tope anom a lies (d13C and d18O) and the trace el e ments in the sed i ments. Such a study has not been car ried out ear lier for this for ma tion.
GEOLOGICAL SETTING
Dur ing the Late Car bon if er ous, the North China Block had reached a po si tion at the east ern end of the Palaeotethys Ocean, form ing the bor der with Panthalassa at its east ern side (Fig. 1A). It did not be long to the supercontinent Pangea (Golonka and Ford, 2000; Shen et al., 2006). The NCB ex pe ri - enced up lift dur ing the Mid dle Or do vi cian Cal edo nian oro gen - esis, ex pos ing the sed i ments of the NCB to long-lived de nu da - tion un til the Late Car bon if er ous. As a re sult, the Mid dle Or do vi - cian to Up per Car bon if er ous suc ces sion is no lon ger pres ent, and the bound ary be tween the Mid dle Or do vi cian and the Up - per Car bon if er ous is an un con formity.
From the Late Car bon if er ous to the Early Perm ian, nu mer - ous lay ers of ma rine lime stone were de pos ited (Wu et al., 1995) (Fig. 1B). The pres ent con tri bu tion fo cuses on a lime stone suc - ces sion in the Huainan Coal Ba sin (HCB; Fig. 1C), de pos ited around the tran si tion from the Car bon if er ous to the Perm ian;
the pre cise age is still a mat ter of de bate. These sed i ments were de pos ited in a cratonic ba sin, at the south east ern mar gin of the NCB.
Dur ing the Late Car bon if er ous, the HCB started sub sid ing, re sult ing in fre quent large ma rine trans gres sions and re gres - sions (Han, 1990; Wu et al., 1995; Wang and Pfefferkorn, 2013;
Shao et al., 2015). A com plete sed i men tary suc ces sion (mostly con sist ing of the Taiyuan For ma tion; the lower part of the Benxi For ma tion is very thin or even lack ing) that con tin ued to the Perm ian thus de vel oped (Wu et al., 1995; Sun et al., 2010). The depositional suc ces sion in the HCB formed at the Car bon if er - ous-Perm ian tran si tion was de vel oped in ma rine-to-ter res trial tran si tional en vi ron ments. It con sists of 11 to 13 lime stone lay - ers with in ter ca la tions of mudstone and sand stone in ter vals, com pris ing thin coal seams (Fig. 2).
The lime stones in the HCB reach a joint thick ness of up to 60 m, which is more than else where on the NCB (Fig. 3). The lime stones in the HCB are over lain by >1500 m of Perm ian, Me - so zoic and Ce no zoic sed i ments.
The fos sils in the Taiyuan For ma tion re flect the po si tion in the con ti nen tal/ma rine tran si tion zone; al ter na tion of trans gres - sions and re gres sions is in di cated by ver ti cal changes in the fre - quency of spe cies with pre ferred hab i tats at dif fer ent depths.
Roughly speak ing, the trans gres sions are re flected in the sed i - ments by the pres ence of fauna, whereas the fos sils in sed i - ments ac cu mu lated dur ing a re gres sion are richer in (washed- in) flora. These in ter pre ta tions are sup ported by thin-sec tion anal y sis of the lime stone sam ples which show var i ous micro - facies, which can be re lated to spe cific con di tions of the deposi - tional en vi ron ment (Flügel, 2010).
It is con sid ered out of scope here to go into de tail with re - gard to the microfacies, as it is well-known that they tend to change rap idly, both lat er ally and ver ti cally, in suc ces sions de - pos ited in an epeiric sea. Con sid er ing the size of the epeiric sea (see Fig. 1B), it is not sur pris ing that the Taiyuan For ma tion also ex tends over hun dreds of kilo metres (Fig. 3), and that a large va ri ety of microfacies is pres ent. We re fer the in ter ested reader to the lit er a ture that fo cuses on this as pect (e.g., Li and Wang, 2006), though for other re gions.
Fusulinids (Quasifusulina sp., Schwagerina sp., Rugoso - fuoulina sp., Triricites sp.) are the most abun dant fos sils, to - gether with cor als (Cyathocarinia maoergouensis, Amplexo - carinia sp., Neokoninckophyllum tortum, Caninophyllum dobro - - lyubovae), cono donts (Streptognathodus elegantulus, Idio - gnathodus hebeiensis, S. elon gates, S. wabaunensis, S.
fuchengensis), cri noids, ammonites and brachi o pod frag ments (Ge et al., 1985; Gao, 1988; Han, 1990; Fan et al., 1999; Wan et al., 2017; Zhang, 2017). The plant re mains (e.g., Neuropteris pseudovata, Annularia cf. orientalis, Lepidodendron posthumii) be long to the typ i cal Cathaysian flo ral realm of China (Hilton et al., 2001; Wang, 2010; Wang et al., 2012).
In spite of the nu mer ous fos sils, the ex act depositional en vi - ron ment of the Taiyuan For ma tion is still con tro ver sial, and sedimentological struc tures and other fea tures have not been stud ied in any de tail thus far, so that lit tle is known yet about the depositional set ting. The fos sils are use ful, how ever, for dat ing, al though also in this re spect some con tro ver sies ex ist. The for - ma tion was pre vi ously con sid ered – mainly on the ba sis of macrofossils – as Up per Car bon if er ous (e.g., Han, 1990), but more re cently it was sug gested – mainly on the ba sis of microfossils – that the mid dle and up per parts of the for ma tion are Lower Perm ian (Asselian), while the lower part would be Up per Car bon if er ous (Kasimovian to Gzhelian) (Wang, 2010;
Wang and Pfefferkorn, 2013).
METHODS
Par tic u larly for the in ves ti ga tion of sam ples re gard ing their geo chem is try and iso tope anom a lies, both the way of sam pling and the an a lyt i cal meth ods are of prime im por tance. Both as - pects are there fore dealt with in the fol low ing sec tions, but – in or der to avoid du pli ca tion of in for ma tion – the reader is also re - ferred to ear lier lit er a ture about the use of trace el e ments and of car bon an ox y gen iso topes for es tab lish ing the depositional en -
vi ron ment, par tic u larly as far as a fresh-wa ter or ma rine set ting is con cerned (see, for a com pa ra ble case study, Yang et al., 2017).
SAMPLING
The Taiyuan For ma tion con tains lime stone lay ers that all have been sam pled and ana lysed in or der to de ter mine the pre - cise con ti nen tal-to-ma rine tran si tional depositional en vi ron - ments of the epicontinental sea in which it was de pos ited. The sam ples were col lected in the Pan’er Coal Mine, which is lo - cated in the north ern part of the HCB. The lime stone sam ples were col lected when bore hole P2 was drilled for coal ex plo ra - tion. The Taiyuan For ma tion in the drilled core has a length of 105 m, in clud ing 13 lay ers of lime stone (Fig. 2); the in di vid ual lime stone lay ers are 0.44–3.58 m thick and they have a cu mu la - tive thick ness of ~50 m. The lime stones were num bered, fol low - ing the prac tice dur ing drill ing, from top to bot tom as L1 to L12;
lime stone L3 was split up into an up per part (L3u) and a lower part (L3l), be cause of a fairly abrupt change from few to many fos sil frag ments and from mod er ate to abun dant cal cite in fill - ings.
It was found dur ing drill ing of bore hole P2 that the Taiyuan For ma tion is sep a rated in this bore hole by a clear un con formity from the un der ly ing Or do vi cian. A black mudstone layer of ~5 m thick forms the top part of the Taiyuan For ma tion (Fig. 2); its na - ture is still not clear. Con sid er ing the com po si tion of the Taiyuan For ma tion, it might be spec u lated that it rep re sents a weath er - ing pro file, re lated to ex po sure that, in turn, might be re lated to the un con formity.
Fig. 1. Lo ca tion map
A – lo ca tion of the study area in its plate-tec tonic con text (af ter Scotese et al., 2005); B – lo ca tion of the study area in the North China Block in pres ent-day China (af ter Wang et al., 1985); C – Huainan Coal Ba sin with lo ca tion
of the Pan’er Mine where the sam ples were col lected
Most of the lime stones of the Taiyuan For ma tion are pale to grey bioclastic micritic wacke- to packstones com posed of abun dant in ver te brate fos sil frag ments. Sev eral lime stone lay - ers con tain ad mix tures of clay (some times car bo na ceous clay), with some intraclasts and ooids.
From the 13 lime stone lev els, 41 sam ples were col lected.
Sam ples with abun dant ad mix tures of ter res trial de bris or with ob vi ous post-depositional (diagenetic) fea tures such as recrystallized cal cite crys tals, cal cite veins or framboidal py rite were ex cluded from the anal y ses. This left 13 sam ples (see Fig.
2) that have been in ves ti gated for their trace el e ments and car - bon and ox y gen iso topes; they are con sid ered rep re sen ta tive for the en tire suc ces sion.
ANALYTICAL METHODS
The sam ples col lected from the 13 lime stone lev els were frag mented into pieces; the out side parts were dis carded, and the frag ments from the in te rior were cleaned ul tra son i cally in pu ri fied wa ter. Some rep re sen ta tive frag ments were pol ished, and thin sec tions were pre pared from other rep re sen ta tive frag - ments in or der to in ves ti gate their mor pho log i cal and struc tural char ac ter is tics by op ti cal mi cros copy (re flected light) and by light-scat ter ing scan ning elec tron mi cros copy (SEM, XL- 30ESEM; Fig. 4). This sedimentology-ori ented in ves ti ga tion is, how ever, not the ob ject of the pres ent study (see the Ob jec tives Fig. 2. Sche matic suc ces sion and char ac ter is tics of the Taiyuan For ma tion as found
in coal-ex plo ra tion bore hole P2 in the Huainan Coal field (mod i fied af ter Wu and Zhang, 2019)
of the study) and is con se quently dealt here with only by pro vid - ing some rep re sen ta tive pho to mi cro graphs (Fig. 4) which pro - vide a gen eral pic ture rather than sedimentological de tails. The chem i cal anal y ses were car ried out fol low ing the GB/T 14506.30- 2010 stan dard (AQUSIC, 2010). The fresh and clean lime stone sam ples were crushed and sieved to ob tain par ti cles of <200 mm. Ap prox i mately 200 mg of the pow dered sam ples were dis solved in a mix ture of the ac ids HNO3, HCl and HF (ra - tio 3:1:1) in a mi cro wave oven.
The re sul tant so lu tions were di luted and spiked fol low ing the Chi nese na tional stan dard GB/T14506.30-2010 (AQUSIC, 2010) for trace-el e ment anal y ses by in duc tively cou pled plasma mass spec trom e try (ICP-MS, X se ries II) at the lab o ra - tory of the Uni ver sity of Sci ence and Tech nol ogy of China at Hefei (prov. Anhui). The rel a tive in ac cu ra cies of these mea - sure ments are <5% for most of the el e ments, as eval u ated by anal y sis of cer ti fied ref er ence ma te rial.
The ox ides of ma jor el e ments (SiO2, Al2O3, CaO, K2O, Na2O, Fe2O3, MnO, MgO, TiO2, and P2O5) were ana lysed by
X-ray flu o res cence spec trom e try (XRF) us ing the method pro - posed by Kimura (1998). The re sults are shown in Ta ble 1.
For d13C and d18O mea sure ments, sam ple pow ders were mixed with pure phos pho ric acid in vac uum at 25°C. The CO2
that was pro duced was mea sured with a Finnigan MAT-252 mass spec trom e ter. The iso to pic re sults are de noted in the pres ent con tri bu tion in the form of delta no ta tion (d13C, d18O – in
‰), rel a tive to the Vi enna Pee Dee Bel em nite (VPDB) stan dard (Coplen et al., 1983), which has a value of 0.011180. The un - cer tain ties of both d13C and d18O are <0.1‰.
RESULTS
As in di cated above, the main an a lyt i cal meth ods were con - cen tra tion mea sure ments of some trace and ma jor el e ments, and de ter mi na tion of the car bon and ox y gen iso topes. The re - sults of these anal y ses are pre sented in the fol low ing sec tions.
Fig. 3. To tal thick ness of the lime stones in the Taiyuan For ma tion (mod i fied from Wu et al., 1995) The source ar eas rep re sent el e vated ar eas that un der went ero sion and sup plied clastics to the epeiric sea
in which the Taiyuan Formation ac cu mu lated
MAJOR AND TRACE ELEMENTS
The most com mon ox ide is CaCO3 (52–95%); the other ox - ides are com monly <5%, ex cept for MgO in sev eral lay ers (Ta - ble 2). The XRD spec tra in di cate that cal cite is the ma jor min - eral, ac com pa nied by rel a tively large amounts of quartz in the
sam ple from lime stone L7 (7.2% SiO2) and do lo mite in L12 (20% MgO).
The vari a tions of re dox-sen si tive el e ments such as Cd, Cr, Mo, and V in the lime stones are pos i tively cou pled with each other, but show a neg a tive cor re la tion with d13C (Fig. 5). Ex cept for Sc (0.41 ±0.11 µg/g, 1s), the other el e ments that are not sol -
A B
C D
E F
Fig. 4. Mi cro pho to graphs of the Taiyuan For ma tion lime stones
A – un iden ti fi able foraminifers (op ti cal mi cro scope, re flected light), sam ple L5 (= from lime stone layer L5); B – un iden ti fi able foraminifers (prob a bly fusulinids) (op ti cal mi cros copy, re flected light), sam ple L8; C – recrystallized cal cite (op ti cal mi cro scope, re flected light), sam ple L7; note the microfossils in the cen tre; D – recrystallized cal cite (op ti cal mi cro scope, re flected light), sam ple L12; note the microfossils in the cen tre; E – framboidal py rite in clu sions (SEM, back scat ter light), sam ple L6; F – framboidal py rite in clu sions (SEM, back scat ter light), sam - ple L9
u ble in sea wa ter, such as Al (0.5 ±0.06%, 1s), Th (1.05
±0.04 µg/g, 1s), and Zr (2.07 ±0.24 µg/g, 1s), have higher con - cen tra tions than in pres ent-day car bon ates.
CARBON AND OXYGEN ISOTOPES
The stud ied sec tion of cores from the Huainan Coal Ba sin shows that d13C val ues vary be tween –1.52 to +2.98‰, thus cov er ing a range of 4.5‰. The range of d18O val ues is from –11.87 to –4.97‰, thus cov er ing a range of 6.9‰ (Ta ble 1).
Sig nif i cant neg a tive ex cur sions in d13C val ues (–1.5‰ for sam - ple L12, –1.1‰ for L7 and –0.8‰ for L2) have been found for sev eral lime stone lay ers, with de creas ing anom a lies from bot - tom to top. Un com monly high pos i tive ex cur sions have been found only in lay ers L4 to L3d (2.3–2.98‰). The d13C val ues in the re main ing lay ers vary only slightly (0.27–1.48‰).
In con trast to d13C, the d18O val ues show a de creas ing trend from layer L12 (–4.97‰) to L7 (–10.30‰), then slightly drops to –8.89‰ in layer L6, be fore fi nally sta bi liz ing within a nar row range (–11.87 to –10.30‰ for lay ers L5 to L1).
ANALYSIS OF THE RESULTS
If the con di tions of the depositional en vi ron ment are to be re con structed on the ba sis of prop er ties of the sed i ment, it is a pre req ui site that changes of these prop er ties due to diagenesis are elim i nated first. Al though the stud ied low-Mg cal cite (Mg
<6.4 µg/g, ex cept in L12) is re sis tant to al ter ation dur ing and af - ter de po si tion (Jacobsen and Kaufman, 1999), we checked whether the sam ples showed ev i dence of diagenetic al ter ation by op ti cal mi cros copy and SEM; we did so also on the ba sis of
T a b l e 2 Trace-el e ments (mg/g) data and anom a lies of the car bon and ox y gen iso topes in the lime stone sam ples from the Taiyuan For ma tion
Sam -
ple Ba Cd Co Cr Mn Mo Ni Sc Sr V U Th Mn/
Sr Th/
U V/Cr Sr/Ba d13C [‰] d18O
[‰] Z
L1 68.13 0.07 0.48 26.64 390.02 0.13 3.63 0.49 751.89 13.43 1.12 2.91 0.52 2.59 0.50 11.04 0.81 -10.43 123.8 L2 124.84 0.76 1.15 62.86 340.05 2.73 25.31 1.61 788.72 34.90 1.01 2.34 0.43 2.32 0.56 6.32 -0.81 -10.33 120.5 L3u 26.81 0.14 0.21 25.09 278.03 0.21 3.45 0.12 537.96 8.93 0.85 2.11 0.52 2.48 0.36 20.06 0.35 -10.30 122.9 L3d 21.41 0.08 0.21 17.86 283.70 0.14 2.27 0.34 366.19 5.71 0.87 2.14 0.77 2.45 0.32 17.11 2.98 -11.13 127.9 L4 18.32 0.09 0.15 14.36 246.25 0.24 2.14 0.22 367.37 4.33 0.31 2.47 0.67 7.93 0.30 20.05 2.30 -11.60 126.2 L5 12.65 0.16 0.08 11.29 337.56 0.27 2.42 0.05 585.34 3.38 0.73 3.02 0.58 4.14 0.30 46.29 1.48 -11.87 124.4 L6 79.98 0.23 0.59 16.54 190.62 0.52 6.59 0.23 495.24 7.45 0.54 1.82 0.38 3.37 0.45 6.19 0.27 -8.89 123.4 L7 36.05 0.53 0.19 44.84 197.46 1.22 2.99 0.15 600.14 13.40 0.71 2.42 0.33 3.41 0.30 16.65 -1.06 -10.30 119.8 L8 23.66 0.14 0.28 21.30 742.85 0.71 5.08 0.57 397.98 5.85 1.24 2.85 1.87 2.30 0.27 16.82 0.75 -10.07 123.8 L9 18.36 0.21 0.24 18.60 253.66 0.36 4.37 0.37 246.97 7.13 1.08 4.23 1.03 3.92 0.38 13.45 0.58 -9.30 123.9 L10 5.70 0.18 0.09 13.89 93.31 0.68 2.16 0.14 248.76 4.11 1.31 3.19 0.38 2.43 0.30 43.67 1.08 -8.00 125.5 L11 13.85 0.14 0.48 18.12 87.35 0.81 6.29 0.72 212.87 8.81 1.64 3.31 0.41 2.02 0.49 15.37 0.84 -6.80 125.6 L12 8.73 0.67 0.91 127.44 188.57 1.63 7.46 0.80 54.29 52.92 1.26 4.05 3.47 3.22 0.42 6.22 -1.52 -4.97 121.7
Sam ple MgO CaO SiO2 P2O5 FeO Al2O3 Na2O CO2 To tal
L1 1.3 50.4 0.1 0.1 1.1 0.4 0.2 46.4 100
L2 10.5 36.3 0.1 0.2 7.2 1.1 0.1 44.5 100
L3u 2.1 49.7 0.2 0.1 1.4 0.8 0.3 45.4 100
L3d 10.6 35.1 0.1 0.2 5.6 1.4 0.1 46.9 100
L4 1.6 50.8 0.3 0.3 1.2 0.6 0.6 44.6 100
L5 0.8 53.1 0.1 0.1 0.5 0.5 0.1 44.8 100
L6 9.4 37.4 0.4 0.2 6.4 1.9 0.1 44.2 100
L7 1.8 47.2 7.2 0.2 1.6 1.2 0.5 40.3 100
L8 2.3 49.6 0.4 0.3 1.2 0.7 0.4 45.1 100
L9 1.8 49.8 0.3 0.1 1.7 1.1 0.3 44.9 100
L10 2.6 49.4 0.2 0.4 1.3 0.8 0.1 45.2 100
L11 1.8 51.1 0.1 0.2 1.2 0.9 0.3 44.4 100
L12 20.2 29.1 0.4 0.4 4.6 1.5 0.1 43.7 100
pre vi ously es tab lished geo chem i cal cri te ria (Frimmel, 2009), par tic u larly the Mn/Sr ra tio. Also the d13C and d18O val ues were mea sured and ana lysed. These as pects will there fore be dis - cussed in the fol low ing sec tions.
DIAGENETIC CHANGES OF THE LIMESTONES
The ex am i na tion of thin-sec tions of all 13 lime stone lev els ver i fied by SEM scan ning con firmed that most sam ples con tain clean cal cite prisms with smooth bound aries, de spite mi nor al - ter ations in sev eral spec i mens (Fig. 6); it must thus be de duced that the diagenetic al ter na tions of the lime stones were min i mal.
The orig i nal sed i men tary lam i na tion found by Han (1990) ap - peared to be well-pre served, which also in di cates that al ter ation or recrystallization was min i mal or even ab sent.
Mn/Sr RATIO
The Mn/Sr ra tio is a com monly used proxy be cause it re - flects diagenetic pro cesses. Stron tium is sen si tive to me te oric diagenesis and is eas ily ex pelled from ma rine car bon ate (Veizer, 2013). Af ter sed i men ta tion, car bon ate rocks will suf fer Sr and Na losses, while be com ing en riched in Fe and Mn, par - tic u larly un der the in flu ence of per co lat ing at mo spheric wa ter (Frimmel, 2009). The Mn/Sr ra tio is, con se quently, a sen si tive in di ca tor to judge the diagenesis and al ter ation de gree of ma - rine car bon ate rocks. An Mn/Sr ra tio of <10 is com monly con - sid ered (e.g., Gao, 1988; Neuhuber et al., 2007; Ganai et al., 2018) as ev i dence that the car bon ate rocks have not been strongly al tered, and that their iso to pic com po si tion still re cords the orig i nal sed i men tary com po si tion. An Mn/Sr ra tio of <2–3
even in di cates that a sam ple has well main tained the iso to pic com po si tion of the orig i nal sea wa ter (Grossman et al., 2002).
The Mn/Sr ra tios mea sured for our sam ples are all <2, ex - cept for the low est lime stone layer (L12), for which Mn/Sr = 3.47. This ex cep tion ally high value is due to an ex tremely low Sr con tent (54 ver sus 213–789 µg/g in the other lime stone sam - ples, Ta ble 1). This sug gests diagenetic al ter ation of the L12 lime stone. This ex cep tional char ac ter of sam ple L12 is sup - ported by the Mg (12.1%), Th (4.05 µg/g), Al (0.79%) and d18O (–4.97‰) val ues of this sam ple, which are sig nif i cantly higher than in the other lime stone sam ples (Ta ble 1).
It must con se quently be de duced that the Mn/Sr ra tios of the lime stones in di cate that the com po si tion of the orig i nal sea wa ter is still re flected by all lime stone sam ples, ex cept L12 (Fig.
5). Apart from the lat ter sam ple, diagenetic al ter ation thus did not play a role of any im por tance.
CORRELATION BETWEEN d18O ANDd13C
The pri mary d18O and d13C val ues of car bon ates in her ited from sea wa ter usu ally de crease af ter burial as a re sult of the per co la tion of me te oric wa ter and/or hy dro ther mal flu ids, and thus can in di cate the de gree of diagenesis. A pos i tive cor re la - tion be tween d13C and d18O makes al ter ation of car bon ates likely. The lime stone sam ples un der study show a cor re la tion be tween d13C and d18O, in deed (r = 0.52, Fig. 6), al though they have lower d18O val ues (–11.9 to –5.0‰) than rep re sen ta tive Car bon if er ous ma rine car bon ates else where in the world, which are ~–3‰ (Brand, 1982; Algeo and Maynard, 2004; Kuleshov et al., 2018), and also lower than Car bon if er ous ma rine lime - stones pres ent on the South China Block, which have val ues in the range of –6.5 to –4.5‰ (Zhao and Zheng, 2014).
A B
Fig. 5. Cor re la tion of d13C (in ‰) ver sus Mn/Sr ra tio (squares) and d18O (in ‰) ver sus Mn/Sr ra tio (dots)
Note that sam ple L12 was prob a bly al tered by diagenetic pro cesses
It can thus be de duced that the d18O and d13C val ues of the Taiyuan lime stone sam ples do not show a clear pos i tive cor re - la tion, in di cat ing that the lime stones pre served the car bon- and ox y gen-iso tope com po si tions of the sea wa ter in which they ac - cu mu lated.
IMPLICATIONS OF THE d18O VALUES
The ox y gen-iso tope com po si tion of car bon ate rocks is sen - si tive to al ter ation. The ini tial d18O value of cal car e ous sed i - ments will, how ever, be dis tinctly re duced dur ing diagenesis be cause of in ter ac tion be tween the sed i ment and per co lat ing wa ter. Quan ti ta tive re search is, how ever, in con clu sive. A value of d18O <–5.0‰ in car bon ate rock in di cates, as a rule, that the car bon ates were af fected by al ter ation; of d18O <–10.0‰, the car bon ates have un der gone strong al ter ation, and the ox y gen- and car bon-iso tope data of the sam ple can con se quently not be used (Han, 1990).
Kaufman et al. (1993) came to the con clu sion that d18O
<–11.0‰ should be taken as the bound ary value. Con se quently, the lime stone sam ples L3d, L4 and L5 would, fol low ing these au - thors, have un der gone diagenetic al ter ation. This would im ply that the car bon- and ox y gen-iso tope data of these three sam ples are no re li able in di ca tors for the sed i men tary en vi ron ment.
Be cause sam ples L3d, L4, L5, and L12 may well have been al tered by diagenesis, they are not in cluded in the other con sid - er ations that lead to the con clu sions of the pres ent study.
d18O AND d13C CHEMOSTRATIGRAPHY
The lime stones of the Taiyuan For ma tion were de pos ited in an epicontinental sea, and they should re cord geo chem i cal in - for ma tion re flect ing the na ture of the terrigenous in put and the com po si tion of the sea wa ter. Com pared to con tem po rary car - bon ate sam ples from other re gions, for in stance in the form of brachi o pod shells from the Palaeotethys Ocean and Pantha - lassa (Bruckschen et al., 1999; Mii et al., 2001) and lime stones
from the South China Block (Zhao and Zheng, 2014), the d18O val ues of the lime stone sam ples un der study here (–11.87 to –4.97‰) are lower. This might be as cribed to dif fer ences in their palaeo geo graphi cal zonation, depositional con di tions, and the cal car e ous spe cies that were ana lysed.
The ox y gen-iso tope com po si tion in ma rine car bon ates is con trolled by sev eral in ter re lated fac tors such as gla cial/inter - gla cial cy cles, fluc tu a tions of the sea level, and the sea wa ter tem per a ture (Ross and Ross, 1988). The de crease in d18O from bot tom to top in the suc ces sion of the Taiyuan lime stone lay ers in di cates a con tem po rar ily low ered sea wa ter d18O, prob a bly caused by grad ual cli mate warm ing and the con se quent sea level rise.
The d13C of an ocean sys tem is in flu enced by the pres ence of or ganic mat ter, dis solved car bonic ac ids, nu tri ent sup ply, pCO2, bur ied or ganic car bon, and sea level fluc tu a tions (McConna ughey, 1989; McConnaughey et al., 1997). The bu - rial of or ganic car bon and stron ger pho to syn the sis will com - monly cause an in crease of the d13C in both sea wa ter and car - bon ate sed i ments. The d13C in the lime stone un der study shows three neg a tive anom a lies in the lower (L2), mid dle (L7) and up per (L12) parts of the suc ces sion; the val ues for these three lay ers are –1.1, –0.8 and –1.5‰, re spec tively. These neg a tive ex cur sions in d13C are pos si bly re lated to short-lived en hanced terrigenous weath er ing in a warm cli mate.
The three just-men tioned lime stone lay ers (L2, L7 and L12) also have the high est con cen tra tions of re dox-sen si tive el e - ments, viz. Cd, Cr, Mo and V. These el e ments tend to be en - riched in sed i ments that ac cu mu lated in an ox y gen-de pleted en vi ron ment. It is con se quently rea son able to as sume that these three lime stone lay ers were de pos ited in an epi - continental ocean with a rel a tively high sea level. Thin-sec tions from these three lime stone lay ers shows less fos sil frag ments than the thin-sec tions from other lay ers. This low amount of biogenic frag ments in the lime stones also sug gests wors en ing con di tions for the biotope that was pre vi ously used to clear wa - ter and a low sed i ment sup ply, but that now be came con fronted with an en hanced terrigenous in put.
Fig. 6. Cor re la tion be tween the d13C (ver ti cal axis, in ‰) and d18O val ues (hor i zon tal axis, in ‰) of the lime stones in the Taiyuan For ma tion
The lime stones L2, L7 and L12, which have anom a lous val ues, are in di vid u ally in di cated
In con trast, the sam ples from lay ers L3d and L4 show pos i - tive anom a lies of d13C, with peak val ues of 3.0 and 2.3‰, re - spec tively. These two lime stone lay ers con tain more shell frag - ments than the other lay ers, sug gest ing less terrigenous in put and thus en hanced or ganic car bon burial. It is im por tant in this con text that d13C in lime stone is more sen si tive to cli mate changes than d18O.
INTERPRETATION OF THE DEPOSITIONAL ENVIRONMENT
Car bon- and ox y gen-iso tope com po si tions pro vide in di ca - tions about the sa lin ity of the aque ous depositional en vi ron - ment. The re dox con di tions of the en vi ron ment in flu ence the sol u bil ity of re dox-sen si tive el e ments in the sea wa ter, and can en rich or im pov er ish these el e ments in the sed i ment. Hence, con cen tra tions or ra tios of spe cific re dox-sen si tive trace el e - ments are com monly used to re con struct the re dox con di tions in an cient sed i men tary sys tems (e.g., Dean et al., 1999;
Yarincik et al., 2000; Morford et al., 2001; Pailler et al., 2002).
Sev eral of these prox ies are there fore pre sented and dis - cussed be low in or der to re con struct the en vi ron ment in which the Taiyuan Formation was de pos ited.
SALINITY OF THE DEPOSITIONAL ENVIRONMENT
Two prox ies are com monly used to in ter pret the sa lin ity if an an cient aque ous depositional en vi ron ment. These con cern the d13C and d18O val ues, and the Sr/Ba ra tio.
SALINITY AS DEDUCED FROM THE d13C AND d18O VALUES
The bench mark study by Keith and Weber (1964) pro posed cor rectly, as con firmed in later stud ies (see also Latal et al., 2004; Guo et al., 2019), an em pir i cal equa tion for es ti mat ing the sa lin ity of aque ous depositional en vi ron ments by us ing d18O (with re spect to SMOW – Stan dard Mean Ocean Wa ter, in ‰) and d13C (with re spect to PDB – Pee Dee Bel em nite, in ‰):
Z = 2.048 × (d13C + 50) + 0.498 × (d18O + 50)
where: Z – palaeosalinity.
If Z >120, the car bon ate was prob a bly de pos ited in a high-sa lin ity ma rine en vi ron ment; if Z <120, de po si tion took place in a low-sa lin ity ma rine or a non-ma rine (lac us trine) en vi - ron ment. The Z val ues have been cal cu lated for all Taiyuan lime stone lay ers and were found to vary from 119.8 to 128.0, sug gest ing that they were pre dom i nantly de pos ited in a high- sa lin ity ma rine en vi ron ment. The L7 lime stone layer, which con - tains abun dant quartz par ti cles, has the low est Z value (119.8), sug gest ing that it was pos si bly de pos ited in a tran si tional (con ti - nen tal-to-ma rine) en vi ron ment (Fig. 6).
SALINITY AS DEDUCED FROM THE Sr/Ba RATIO
Sr and Ba rep re sent a unique group of co her ent el e ments with sim i lar geo chem i cal prop er ties. Their dis tri bu tions vary closely with sa lin ity, and are more eas ily con cen trated in sea
wa ter than fresh wa ter (Schmitz et al., 1997). Sev eral sed i men - tary en vi ron ments can af fect the Sr/Ba ra tio, due to the dif fer - ences in chem i cal be hav iour of these two el e ments.
The sol u bil ity of Ba com pounds is dis tinctly lower than that of Sr com pounds, so that Ba com monly pre cip i tates as BaSO4 if sul phate is pres ent in the wa ter (Van Os et al., 1991). The Sr/Ba ra tio can con se quently be used as an in di ca tor to es ti mate the ma rine or con ti nen tal (mostly lac us trine) na ture of the depositional en vi ron ment (Van Beek et al., 2003).
An Sr/Ba ra tio of 1 is con sid ered as the bound ary be tween fresh (Sr/Ba <1) and ma rine (Sr/Ba >1). The Sr/Ba ra tio through out the lime stone suc ces sion re mains within the range be tween 6.2 and 46.2, which is clear ev i dence of a ma rine depositional en vi ron ment.
REDOX CONDITIONS AS DEDUCED FROM GEOCHEMICAL PROXIES
The con cen tra tions of sev eral chem i cal el e ments are, as well as the ra tios be tween some el e ments, prox ies for the re dox con - di tions dur ing (and di rectly af ter) sed i men ta tion, as dis cussed above. The most in for ma tive prox ies are the com bi na tion of Cd and Mo con cen tra tions, the V/Cr ra tio, and the Th/U ra tio.
REDOX CONDITIONS AS DEDUCED FROM THE Cd AND Mo CONCENTRATIONS
Cd and Mo are both re dox-sen si tive trace met als, and their sol u bil ity un der ox i diz ing con di tions is higher than un der re duc - ing con di tions, con se quently re sult ing in their rel a tive en rich - ment in ox y gen-de pleted sed i men tary fa cies (Rosenthal et al., 1997; Adelson et al., 2001). It is com monly agreed upon that car bon ates are de pos ited in an ox y gen-de pleted en vi ron ment if the con tents of Cd and Mo are higher than 1 µg/g and 8 µg/g, re - spec tively (e.g., Chaillou et al., 2002; Sundby et al., 2004).
In most of our sam ples, Cd and Mo are pres ent in con cen - tra tions lower than 0.2 and 1.0 µg/g, re spec tively (Fig. 7). This in di cates that al most all lime stones were de pos ited in an ox i diz - ing en vi ron ment.
REDOX CONDITIONS AS DEDUCED FROM THE V/Cr RATIO
Cr is com monly pres ent as CrO42- in ox i diz ing en vi ron ments, and as the re duced form (+3 va lence) in re duc ing en vi ron - ments. V be haves sim i larly: it is well-sol u ble in an ox i diz ing en - vi ron ment (+6 va lence, mainly as H2VO4–) and less sol u ble un - der re duc ing con di tions (Hastings et al., 1996).
Jones and Man ning (1994) sug gested that Cr is of ten in - cor po rated in clastic par ti cles, for ex am ple, as a sub sti tute for alu mi num in clay min er als, but it can also be ab sorbed or pres - ent as chro mite. Va na dium is con strained by or ganic mat ter, and is con cen trated in re duc ing en vi ron ments (Jones and Man ning, 1994; Tribovillard et al., 2006). Jones and Man ning (1994) de tailed that V/Cr <2 in di cates an ox i diz ing depositional en vi ron ment, that 2< V/Cr <4.25 in di cates some - what ox i diz ing con di tions, and that V/Cr >4.25 is ev i dence of re duc ing con di tions.
The V/Cr ra tios of the Taiyuan lime stone sam ples hardly vary, show ing dis tinctly low val ues, com monly <0.6 (Fig. 7).
This in di cates pro nounced ox i diz ing con di tions of the sea wa ter when the lime stones were de pos ited.
REDOX CONDITIONS AS DEDUCED FROM THE Th/U RATIO
Ura nium ions main tain a high re dox state (U+6) un der ox i diz - ing con di tions, form ing sol u ble ura nyl car bon ates in sea wa ter.
Un der re duc ing con di tions, how ever, U is pres ent in a lower re - dox state (U+4), form ing in sol u ble ura nous flu o ride, and is trapped into ma rine car bon ates (McManus et al., 2005). Th is less sen si tive than U to fluc tu a tions in the re dox con di tions and re mains per ma nently in its in sol u ble Th+4 state. This im plies that sed i ments de pos ited in re duc ing en vi ron ments are rel a - tively rich in U and thus have lower Th/U val ues than those in ox i diz ing en vi ron ments (Sarin et al., 1990). It was sug gested (Pailler et al., 2002) that Th/U <2, 2< Th/U <7 and Th/U> 7 in sed i ments in di cate re duc ing, ox i diz ing and strongly ox i diz ing sed i men tary en vi ron ments, re spec tively.
The Th/U ra tios in the Taiyuan lime stones vary be tween 2 and 5, ex cept for a peak in L5 (Th/U = 7.9). This in di cates over - all ox i diz ing con di tions (Fig. 8).
DISCUSSION
The Up per Car bon if er ous to Lower Perm ian Taiyuan For - ma tion has for a long time raised hot de bates about its depositional en vi ron ment and nu mer ous ar gu ments have been put for ward, both in fa vour and against ma rine and con ti nen tal set tings (e.g., Han, 1990; Guo and Liu, 2000; Li and Wang, 2006; Song et al., 2011, 2016; Mao et al., 2018; Wang et al., 2019). Part of the con tro versy is based on dif fer ent views re - gard ing the fos sil con tent. Ter res trial plant re mains have been found, to gether with spe cies rep re sent ing a full-ma rine fauna such as ammonites and cri noids; nu mer ous microfossils have been found as well, many of which in di cate a tran si tional con ti - nen tal-to-ma rine en vi ron ment. This might, in prin ci ple, be ex - plained by fa cies shifts in a near-coast en vi ron ment, but field data do not pro vide any in di ca tion for such fa cies shifts.
Geo chem i cal and iso tope anal y ses of sev eral chem i cal el e - ments have there fore been car ried out be cause they have been found in the past de cades to be fairly re li able prox ies that can help in ter pret the depositional en vi ron ment.
The most im por tant anal y ses car ried out for the pur pose are the anal y ses of the d13C and d18O val ues, and the Sr/Ba ra tio, be cause they pro vide in sight into the sa lin ity of the depositional en vi ron ment. Both prox ies in di cate a full-ma rine en vi ron ment (apart from, pos si bly, one lime stone that may have been de pos - ited in a tran si tional en vi ron ment). The com mon out come for both prox ies sup ports this find ing.
Equally con sis tent find ings were ob tained re gard ing the re - dox con di tions of the depositional en vi ron ment. The three prox - ies that were de ter mined (Cd and Mo con cen tra tions, the V/Cr ra tio, and the Th/U ra tio) all yield the same out come: an ox i diz - ing en vi ron ment. This is con sis tent with the full-ma rine set ting ob tained through the prox ies for the sa lin ity.
The con sis tency be tween all out comes must be con sid ered as strong ev i dence that the find ings are cor rect, even though the ac cu ra cies of the var i ous prox ies in them selves may be ques tioned.
Ob vi ously, this un avoid able con clu sion raises the ques tion how the ter res trial fos sils in the Taiyuan For ma tion can be ex - plained. There are sev eral pos si ble ex pla na tions. One might be that ter res trial plants have been eroded from a coast or, more likely, have been sup plied by rivers that dis charged in the sea. It is well-known that plants may drift away from their source over long dis tances in oceans. It seems un likely, how ever, that the fairly fre quent re mains of ter res trial plants in the ma rine sed i - ments of the Taiyuan For ma tion have such an or i gin. A sec ond pos si bil ity is that the plants were brought along by sed i ment grav ity flows. Al though it has been found that such mass flows can oc cur even in the mostly fairly shal low en vi ron ment of epicontinental seas (e.g., Chen et al., 2009; Van Loon et al., 2012; Su et al., 2016; Wang et al., 2020), this seems also un - likely, as no clear in di ca tions of mass trans port have been de - scribed from this for ma tion.
Fig. 7. Ver ti cal vari a tions in the d13C and d18O val ues (in ‰) and in the Cd, Cr, Mo and V con cen tra tions in the Taiyuan lime stone sam ples
A so lu tion for the “prob lem” of the ter res trial fos sils may be found in the ex pla na tion pre sented for an other Chi nese for ma - tion that was hotly de bated for its ma rine or con ti nen tal na ture for a long time: the Cre ta ceous Lingshandao For ma tion on Lingshan Is land in the Yel low Sea, 37 km south of Qingdao.
This for ma tion was re cently also in ves ti gated for its geo chem is - try (Yang et al., 2017), and it turned out that the geo chem i cal prox ies in di cate clearly that the set ting was ma rine. Yet there were, like in the Taiyuan For ma tion, ter res trial (car bon ized) plant re mains and sam ples of which the geo chem is try in di cated brack ish, low-sa lin ity con di tions.
Anal y sis of the sed i men tary fa cies of the Lingshandao For - ma tion in di cated that not only sed i ment grav ity flows left turbi - dites and debrites (Yang and Van Loon, 2016; Yang et al., 2016) that brought ter res trial fos sils to the deep-ma rine realm, but that also sud den in fluxes of fresh wa ter dur ing high-dis - charge phases of a river low ered the sa lin ity of the am bi ent sea wa ter tem po rarily. This anom a lous low sa lin ity of the sea wa ter was pre served in the pores of the then de pos ited hyper - pycnites, which are well-rec og niz able be cause of their nu mer - ous cou plets of mm-sized lev els of fin ing-up ward and coars en - ing-up ward suc ces sions, caused by fluc tu a tions in the dis - charge of wa ter and/or sed i ment by the river de bouch ing into the ba sin. Such high-dis charge phases of fresh river wa ter may also ex plain the ap par ent low sa lin ity when lime stone L7 of the Taiyuan For ma tion was ac cu mu lat ing. Hyperpycnites and tur - bid ity cur rents thus can well-ex plain the tem po rary rel a tively low-sa lin ity con di tions of the Taiyuan For ma tion, al though the for ma tion was de pos ited in a full-ma rine set ting.
CONCLUSIONS
Pet ro log i cal anal y ses and geo chem i cal in di ces (d13C ver - sus d18O ver sus Sr/Mn) of lime stone sam ples from the Taiyuan For ma tion in the Huainan Coal Ba sin (North China Block) in di cate that al most all lime stones have been af fected by weak or neg li gi ble diagenetic al ter ation. Three lime stone lay ers are also as so ci ated with the high con cen tra tions of re - dox-sen si tive el e ments (Mo, V, and Cd). These three lay ers are in ter preted to have been de pos ited in a sea dur ing a high sea level phase thanks to a tem po rary warmer cli mate and with more terri genous in put than the other lime stone lay ers.
Two lay ers with pos i tive d13C anom a lies sug gest less terrigenous in put and a higher con cen tra tion of or ganic mat ter in the sed i ment. The trace el e ments and d18O ver sus d13C in di - cate a full-ma rine ox i diz ing set ting. The pres ence of ter res trial plant frag ments and the low-sa lin ity of some lime stone sam - ples must be at trib uted to high-dis charge phases of fresh river wa ter that re sulted in hyperpycnal flows bringing the ter res trial plant ma te rial to the full- ma rine realm and also tem po rarily low er ing the sa lin ity lo cally.
Ac knowl edge ments. This study was fi nan cially sup ported by the Na tional Key R&D Plan of China (Grant No.
2017YFC0601405), the Na tional Nat u ral Sci ence Foun da tion of China (Grant No. 41772096; 41972170) and the SDUST Re - search Fund (Grant No. 2018TDJH101). The com ments by the jour nal re view ers M. Krajewski and Anon y mous are kindly ap - pre ci ated.
Fig. 8. Dis tri bu tion of the Sr/Ba, V/Cr and Th/U ra tios in the Taiyuan lime stone sam ples
AQUSIC (Gen eral Ad min is tra tion of Qual ity Su per vi sion, In - spec tion and Quar an tine of the Peo ple’s Re pub lic of China, Stan dard iza tion Ad min is tra tion of the Peo ple’s Re pub lic of China), 2010. GB/T 14506.30–2010. Meth ods for chem i cal anal y sis of sil i cate rocks – Part 30: De ter mi na tion of 44 el e - ments. AQSIC, Beijing.
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