Estimation of primary productivity in the Toarcian Tethys — A novel approach based on TOC, reduced sulphur and manganese contents

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Abstract

About 60 samples from the non-bioturbated Úrkút Manganese Ore Formation (Bakony Mountains, Hungary), representing the early Toarcian anoxic event, were studied for: (1) TOC content, Rock-Eval parameters and kerogen δ13C; (2) S content and δ34S and (3) Al, Ca, Mn, Si and Ti contents. Using the example of the formation, a novel approach for estimating paleoproductivity is presented, taking into account reduced sulphur and manganese contents, which reflect the amount of organic carbon lost during early burial.

Drastic changes in Mn content (between 0.7% and 37%) during deposition of the formation resulted in very variable early diagenetic processes, with sulphate reduction dominating in low-Mn sediments, and Mn reduction dominating in high-Mn sediments. The amounts of reduced sulphur and manganese allowed calculation of the original organic carbon (TOCor) content throughout the formation. Based on stratigraphic variations in chemical composition, changes in the rate of sedimentation were assessed for the three main members of the formation. Next, using the formula describing the relationship between measured productivity, rate of sedimentation and carbon flux reaching the sea-floor, developed by Suess (1980), past productivity was calculated. The values obtained, lower than those prevailing in present-day upwelling systems, are probably underestimated because of uncertainties of chronostratigraphic and water-depth data. However, relative differences between productivity values obtained for the three members are considered as realistic and suggest an increase in productivity during deposition of the formation. This increase is not related to stratigraphic variations in Mn content and is accompanied by an increase in the growth of calcareous plankton.

Introduction

Worldwide extinctions of marine biota and its subsequent recovery resulted in significant changes in primary productivity in the ancient oceans. Hence assessment of productivity changes is an important aspect of extinction event research. These changes affected the chemical composition of the contemporaneous sediments and it is now possible to study their chemical evolution.

Several chemical tracers have been proposed for estimating paleoproductivity: minor elements (for example Ba, Dymond et al., 1992); the carbonate δ13C of coexisting benthic and pelagic fossils (Broecker and Peng, 1982); the carbon-isotopic composition of marine organic matter (OM) (Shemesh et al., 1993); and the flux rate of biogenic silica (Diester-Haass et al., 1992).

During the last decade, empirical formulae, based on the relationship between flux of organic carbon reaching the sea-floor, rate of sedimentation and measured productivity were developed to estimate the paleoproductivity in oxic (Müller and Suess, 1979; Stein, 1986, Stein, 1991) and anoxic (Bralower and Thierstein, 1984) environments, respectively. In these formulae, loss of organic carbon due to early diagenetic bacterial processes was indirectly correlated with rate of sedimentation. Here an approach is proposed which takes directly into account early diagenetic loss of organic carbon using amounts of reduced sulphur and manganese present in anoxic sediments.

After a short discussion of its principles, the technique will be demonstrated using the example of the Úrkút Manganese Ore Formation, a Tethyan record of the early Toarcian anoxic event.

Section snippets

Principles of productivity assessment

Based on observations made in the Black Sea, Bralower and Thierstein (1984)suggest that, in the case of anoxic sediments, the rate of organic carbon burial represents 5% of the primary productivity. There are serious arguments against the use of the Bralower and Thierstein (1984)approach for estimating productivity in anoxic sediments in general and in Mn-rich anoxic sediments in particular. First, their approach disregards water depth. Then it considers the degradation of OM taking place in

Geologic setting

The Úrkút Manganese Ore Formation, a prominent representative of Lower Toarcian black shale-hosted Mn-carbonate mineralization in the Alpine realm (Jenkyns et al., 1991), is known from small occurrences in the Transdanubian Central Range (Fig. 1). This paper deals with a section exposed in the Úrkút subsurface manganese ore mine, located in the Bakony Mountains, about 150 km to the west of Budapest. The stratigraphy and lithology of the Jurassic sequence at Úrkút are displayed in Fig. 2Fig. 3.

Experimental methods

Our samples, collected from the mine section `Level 1' studied by Polgári et al. (1991), were ground in a Fritsch ball mill. Al, Ca, Mn, Si and Ti measurements were carried out on an Jobin-Yvon JY 70 ICP device after fusion with Li-metaborate.

TOC content, Hydrogen Index (HI) and Tmax were determined by a Delsi Oil Show Analyser. Due to their high carbonate-bound Mn content, samples were suspected of generating CO2 by thermal breakdown of rhodochrosite and Mn-rich calcite during pyrolysis. For

Results

TOC, S contents, HI and kerogen δ13C values are listed in Table 2 and displayed in Fig. 3, Fig. 4. HI varies between 100 and 400 with high values in the marl and low ones in the ore beds. The kerogen is clearly immature, with Tmax varying between 407°C and 431°C. TOC contents are high in the marl and low in the main ore bed, with intermediate values in the II ore bed. Kerogen δ13C values vary between −30‰ and −34‰ (Table 2; Fig. 4). Marl kerogen is characterized by a lighter carbon-isotopic

Preliminary remarks on sediment sources, sea-water oxygenation and early diagenesis

Data in Table 1, Table 2, Table 4 suggest that the Úrkút Manganese Ore Formation had three main sources: clay minerals of terrigenous origin, biogenic material (silica, carbonate and OM), and Mn–Fe minerals of uncertain (submarine hydrothermal?, Kaeding et al., 1983; Polgári et al., 1991) origin. Biogenic silica reached the sea-floor in the form of radiolarian tests. Their mass occurrence suggests high primary productivity (De Wever, 1994) and a significant contribution of radiolarian cell

Conclusions

Drastic changes in the rate of Mn flux during deposition of the Úrkút Manganese Ore Formation led to very different types of early diagenesis with Mn reduction and sulphate reduction being the dominant processes of OM degradation in the main ore bed and the marl, respectively. In the relatively Mn-poor II ore bed both reactions were important. These differences in early diagenesis are evidenced by high TOC and S contents, relatively high HI values and a low Mn content in the marl and low TOC

Acknowledgements

This research was partly supported by the Hungarian Science Foundation (OTKA) under Grant No. 554. Helpful comments from Hugh Jenkyns and Ruediger Stein improved the text. Information from Zoltán Szabó about the geological setting and his help during sampling are gratefully acknowledged. Discussions with A. Mindszenty, M. Polgári, I. Viczián and A. Vörös improved the text. Figures were drafted by Ilonka Honfi.

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