~ • The Paleocene-Eocene boundary in a flysch sequence from Uoriska Brda (Western Slovenia): Oxygen and carbon stable isotope
variations
Paleocensko-eocenska meja v flišu Goriških Brd (zahodna Slovenija): variabilnost
izotopske sestave kisika in ogljika
Tadej DOLENEC,12 Jernej PAVŠIČ,1 & Sonja LOJEN2 Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana 2r, ^ .	.	Aškerčeva 12, 1000 Ljubljana, SlSvenia	JJ'
Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
tope!V W°rdS: P/E b°Undary' N0Zn°' Goriška Brda' flysch- oxygen and carbon stable iso-Ključne besede: meja P/E, Nožno, Goriška Brda, fliš, stabilni izotopi kisika in ogljika
Abstract
Stable (513CCarb., 613C0rg., 8lsO) isotopic records were studied to elucidate the nroblem
W Ä^fe1?^ in 3 flysch secluence at NoznoTn ¿oriška Brfa W. Slovenia. A remarkable disturbance in organic carbon as well as in carbonate C and O isotopic records and associated Ir anomalies provide strong evidence thTtsomething unusual happened at the time of the Paleocene-Eocene transWiol The diverseXracte? of the negative excursion of 813C of carbonate and sedimentary organic carbon inSes a significant perturbation in the global carbon cycle and climatic changes most Drobablv controlled by a combination of volcanic activity and fluctuations in bioproXtivitv The SffiÄ	t0 gl0bal	t0 * higher green-
Kratka vsebina
Z namenom da razjasnimo problematiko paleocensko-eocenske meje v flišu Goriških B£d' r°JaZ1SkaTU potek krivuli 513Ccarb„ i13Corg in 8">0 v mejS flišni sekvenci na območju Nožna. Izrazite motnje v izotopskem zapilu vseh treh parametrov kfsoTOada^o
LirJv°mvllitmltkaŽe]0't.da je prišl° na Preh°du iz paleocena v eocen do nemvad h dogodkov. Večkratna negativna anomalija za karbonatni ogljik in ogljik iz sedimenUrane organske snovi najverjetneje odraža globalne spremembe v ogljikovimciklusu S?klimat Hnl^fvembKe,Te T ^verjetneje posledica vulkanske dejavn^tHn^oTebaniav b opral dukciji. Variabilnost parametra 8180 pa je lahko povezana z globalno otoplitviio zaradi povečanega vpliva efekta tople grede, deloma pa tudi z diagenltskimi spremeSbamf
Introduction
The transition from Paleocene to Eocene was a time of global climatic, océanographie and biotic changes resulting in one of the
largest calcareous deep benthic foraminifer-al extinction events in the last 100 million years (Kennett & Stott, 1991; Pak & Miller,1992;Charisi & Schmitz, 1995; Schmitz et al., 1996). This signi-
ficant short-term event, which is placed in the latest Paleocene between 57.5 and 58 m.y. (Corfield, 1994), coincides with distinct negative 513C and 5180 anomalies, recorded in several marine and terrestrial sections all over the world (C o r f i e 1 d , 1994; C a n u d o et al., 1995; B r a 1 o w e r et al., 1995; K o c h et al., 1995; Charisi & Schmitz, 1995; Lu et al., 1996), as well as in geochemical changes (Schmitz, 1996; Thompson & Schmitz, 1996; Schmitz et al., 1997). Furthermore, Ir anomalies have also been recorded (Schmitz, 1996; Dolenec et al. 1998; Dolenec et al., 2000). A pronounced negative (13C and (180 excursion indicates major changes in the structure of the deep water circulation system and biological productivity, and is probably related to a transient climatic anomaly also named »The Late Paleocene Thermal Maximum« (Z a -c h o s et al., 1993). It remains unclear what triggered such a sudden temperature rise
near the Paleocene/Eocene (P/E) transition. However, several authors proposed that an increase in atmospheric pCC>2 due to the addition of isotopically light C02 from seafloor volcanic activity and/or terrestrial sources (Roberts et al., 1984; Owen & R e a, 1985; R e a et al., 1990; Thomas, 1991), together with outgassing of marine CO2 due to reduced surface water productivity, would account for the temperature rise from the late Paleocene into the early Eocene (Corfield, 1994).
The present study was initiated to determine whether there were any systematic changes in the stable isotope composition of carbonate carbon and oxygen and organic carbon during the P/E boundary transition related to the global events considered. For these purpose the carbonate and sedimentary organic carbon isotopic composition were measured in a flysch sequence from Goriska Brda (Nozno section) in Western Slovenia, in the hope that such a study would provide
Fig. 1. Map showing location of studied region in Goriška Brda.
further information necessary to elucidate the cause of the isotopic anomalies across the P/E transition.
Geological setting and biostratigraphy
On the Dinaric carbonate platform of the western Tethys sea, flysch sedimentation started in the Paleocene and continued up to the Middle Eocene (Buser & Pav-š i č , 1978). A well exposed, non-tectonized P/E sedimentary boundary sequence was found only in Goriška Brda (W. Slovenia, Fig. 1). The lower part of the studied section represents 30 m thick unit of the uppermost Paleocene biozone NP9, mostly composed of clayey marls alternating with sandstones. The Upper Paleocene age of these beds is indicated by calcareous nannoplankton assemblages (Pavšič & Dolenec, 1995). This sedimentary sequence is overlain by a 3 m thick limestone breccia unit followed by an up to 11.8m thick succession of sandstones containing only re-deposited alve-olinids and discocyclinids from older Paleocene beds. The first Eocene plankton
species Rhomboaster bramlettei, indicating the biozone NP 10, was found in the upper part of the first marl layer overlaying the sandstone. It is followed by a 4 m thick normal flysch sequence predominantly composed of marls and sandstones. Based on the lithology and micropaleontological investigations, the P/E boundary is placed arbitrarily at the base of the limestone breccia unit (P a v s i 6, 1997).
Materials and methods
The boundary profile in the flysch succession was systematically sampled in order to determine all the details of the P/E stable isotope anomalies. The relative stratigraphie position of the samples and isotopic results are presented in Fig. 2. Geochemical analyses were carried out at the Activation Laboratories, Ontario. The isotopic measurements were performed at the Jožef Stefan Institute, Ljubljana, Slovenia. Whole rock samples for 813C and 5180 analyses of carbonate were prepared by overnight digestion in > 100 % phosphoric acid at 50 °C.
Fig. 2. Stable isotope composition of carbonates (8130, 513Csarb.), organic carbon (813Corg) and isotope fractionation between carbonate and organic carbon (A5l3Ccarb _org) across the Paleocene-Eocene boundary at Nozno in Goriška Brda, W. Slovenia (O Ir anomaly)
CO2 released during acid treatment was cryogenically cleaned and analyzed using on a Varian MAT-250 mass spectrometer. Organic carbon isotope ratios were determined in the carbonate-free residues in an Europa 20-20 Stable Isotope Analyser (Europa Scientific LTD.) with an ANCA-NT preparation module for on-line combustion of bulk solid samples and chromatographic separation of the gases. All 813C and 8180 values are expressed in standard permil (%o) notation relative to the PDB standard. The analytical precision, based on multiple analysis of internal laboratory standards, was ± 0.01 %o for 813Ccarb. and 813Corg. and ± 0.09 %o for 8laO. Overall analytical reproducibility of the isotope data was better than ( 0.1 %o for 813Ccarb., 813Corg., and for 8180.
Using petrographic and trace element abundance data (unpublished), we evaluated known diagenetic effects that may affect the oxygen and carbon isotopic composition in our studied samples and concluded that the 813C values reported here are predominantly primary. The oxygen isotopic composition of the whole rock samples is lower than those recorded from most coeval marine sections worldwide, most probably indicating diagenetic alteration. However, the low negative correlation (r = -0.46) between S13Ccarb. and S13Corg. (r = -0.46) indicates that the carbon isotopic composition of the whole rock was partially modified by the oxidation of organic matter. Although the post-depositional processes more or less altered the original oxygen and carbon isotopic composition, we believe that the primary paleoceanographic signals were not completely overprinted. This assumption is supported by similar trends of 813Ccarb. and S180 values in whole rock and foraminiferal samples of P/E boundary sections from Egypt (Charisi & Schmitz, 1995; S c h m i t z et al., 1996) and Spain (L u et. al., 1996).
Results and discussion
The short term event recorded in the P/E boundary flysch sequence of the Goriška Brda section is characterized by complex characters of isotopic anomalies which
roughly span the interval between 0 and -5 m. There are four distinct isotopic (S13Ccarb., 513c0rg. and 8180) minima at -0.25, -1.3, -2.5 and -4 m which demonstrate a clear perturbation in both 813Ccarb. and 8180 records of the whole rock samples, as well as in 813C of organic carbon (Fig. 2). The Ir anomalies (Dolenec et al., 2000) at -4m (0.1 ppb), -2.5 m (2.3 ppb) and -1.3 m (0.6 ppb) are coincidental with isotopic anomalies, while that at -2.95 m (0.3 ppb) appears after the first negative 8lsO and 813C shift. The possible link between isotopic and Ir anomalies is not yet well understood. The possibility of an impact triggering a P/E bi-otic crisis and isotopic and Ir anomalies is too speculative. No clear impact signatures have been found in the studied section or in any other P/E section worldwide. A significant decrease of 813Ccarb. from +1.80 to -4.87 (and a negative 8lsO shift from -2.2 to -6.0 (in the whole rock samples is most probably related to a rapid short-term carbon and oxygen isotope excursion associated with the benthic extinction event which occurred close to the NP9/NP10 and P6a/P6b zone boundaries, within the latest Paleocene, as documented on a global scale (Bralower etal., 1995; Lu etal., 1996).
The corresponding organic carbon 813Corg. isotope curve is slightly different with respect to those of 813Ccarb. and 8180, showing a gradual increase in 818C values from -25.82 to -24.13 %o, starting in the upper part of the late Paleocene and lasting through the lower part of the early Eocene. Superimposed on this long term trend is a short-term perturbation of 813Corg. values coinciding with the negative shift in 813Ccarb. and 8lsO, thus indicating some changes in bioproductivity. The 813Corg-curve also exhibits four isotopic minima. The first one at -5 m appears before the first negative 813Ccarb, and 8lsO excursion, while the last three are coeval with those of 813CCarb. and 8180. Hollander et al., (1993) have shown that a decrease in 813Corg. and a subsequent increase in A813Ccarb.-org. could signify a "Strangelove Ocean" wherein dissolved CO2 was enriched in 13C because of reduced plankton production, while increasing 813Corg. and decreasing A813Ccarb.-org. values are consistent with increased oceanic productivity. The diverse
character of the A513Ccarb.-org. and 813Corg. anomalies in the boundary sequence could be related to the fluctuations of primary productivity which would tend to destabilize the global climate and possibly caused significant perturbations in atmospheric
co2.
The short term disturbances in 813Ccarb. and 813C0rg. records in the topmost part of the biozone NP9 can be explained by perturbations in atmospheric pCC>2 levels and/or climatic changes, most probably controlled by a combination of increased sea-floor spreading, volcanic activity (Owen & Rea, 1985; Sloan et al., 1992; Dickens et al., 1995) and fluctuating bioproductivity. The corresponding §180 variability may be related to global warming due to the higher level of greenhouse gases in the atmosphere and/or diagenetic alteration.
Conclusions
The transition from Paleocene to Eocene is characterized by a complex character of isotopic anomalies which demonstrate that something unusual happened at the P/E boundary. Their origin is unclear, but a combination of increased volcanic activity and fluctuating bioproductivity seems to be responsible for a sudden increase of atmospheric pC02, temperature rise and biotic crisis. However, the discovery of an Ir anomaly which is associated with perturbations in isotopic (A813CCarb., 813Corg. and 8lsO) records may also suggests a link between volcanic processes and/or the highly speculative possibility of extraterrestrial dust contamination. However, our results are inconclusive and further more detailed and complex geochemical studies are required to resolve this problem.
Acknowledgements
This study was financially supported by the Ministry of Science and Technology, Republic of Slovenia, and Geoexp d.o.o. Tržič, Slovenia. To both these institutions we express our sincere thanks.
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