Elsevier

Organic Geochemistry

Volume 29, Issues 5–7, November 1998, Pages 1649-1662
Organic Geochemistry

Molecular indicators of redox and marine photoautotroph composition in the late Middle Ordovician of Iowa, U.S.A.

https://doi.org/10.1016/S0146-6380(98)00185-5Get rights and content

Abstract

Saturated and aromatic hydrocarbons were used to evaluate depositional redox conditions and marine photoautotroph contributions to Middle Caradocian strata of the central United States (IA). At the base of the Spechts Ferry Member of the Decorah Formation, 13C-enriched aryl isoprenoids, derivatives of green sulfur bacteria, become abundant, indicating the development of photic-zone anoxia. This is coincident with the disappearance of the organic-walled microfossil Gloeocapsomorpha prisca and a marked decrease in the relative abundances of cyanobacterial biomarkers. The development of dysoxic to anoxic conditions and/or associated changes in basin circulation potentially affected the distributions and abundances of these organisms. In the overlying Guttenberg Member, G. prisca-derived organic matter becomes dominant, but relative cyanobacteria abundances remain low. In addition, the percentage total organic carbon is greater than 20%, even though selected biomarker ratios (pristane/phytane ratios greater than 3, and homohopane indices less than 0.5) and the presence of bioturbation indicate that bottom waters were oxygenated. It is suggested that deposition of G. prisca affected both organic matter preservation and depositional redox conditions. Observed variations in redox indicators and marine photoautotroph contributions are associated with changes in siliciclastic deposition, reported macrofauna turnover and with evidence for oceanic cooling and a change in circulation patterns documented in the eastern United States.

Introduction

The late Middle Ordovician (P. undatus zone) of the eastern United States contains significant evidence for macrofaunal turnover (Patzkowsky and Holland, 1993, Patzkowsky and Holland, 1996, Patzkowsky and Holland, 1997; Frey, 1995). These authors suggested that a tectonically driven change in basin circulation and/or increased upwelling caused cool, oxygen-poor waters to spread through the Taconic foreland basin, resulting in the extinction of fauna in the eastern United States (Patzkowsky and Holland, 1996). Evidence for profound variation in basin circulation in this interval includes a transition from tropical- to temperate-type carbonates, a decline in the abundances of calcareous green algae and cyanobacterial mats and an increase in phosphorite deposits (Patzkowsky and Holland, 1993; Holland and Patzkowsky, 1996, Holland and Patzkowsky, 1997). A pronounced positive carbon-isotope excursion in both carbonate (∼3‰) and bulk organic matter (3–7‰) spans the interval of faunal change (Hatch et al., 1987; Ludvigson et al., 1996; Patzkowsky et al., 1997) and offers further evidence of regional and possibly global changes in carbon cycling.

During the same interval, a significant local extinction occurs in strata from the upper Mississippi valley (Sloan, 1987, Sloan and Alexander, 1997), suggesting that oceanographic processes similar to those of the eastern United States affected biota in the central United States. Indeed, in correlative units in eastern Wisconson Saylor et al. (1997)find a lithofacies shift to cool-water-type carbonates similar to those observed in the eastern U.S. However, Sloan (1987)invoked deposition of a widespread volcanic ash (the Deicke K-bentonite) rather than paleoceanographic processes as the cause of extinction.

Here, we use molecular indicators to examine the timing and magnitude of redox variations related to these oceanographic changes in a single continuous core from Iowa.

Specifically, we evaluate whether redox variations are associated with published evidence for bioturbation, lithofacies variations and inferred water depth. A second goal is to evaluate changes in the organic materials derived from photosynthetic organisms using molecular markers and to determine if those changes track sedimentological evidence for redox or water-depth variations. In particular, the contributions from cyanobacteria, anoxygenic photosynthetic bacteria, and Gloeocapsomorpha prisca are evaluated using lipid biomarkers. To achieve these goals, we performed molecular and isotopic analyses on saturated and aromatic hydrocarbons isolated from a continuous core of the upper Platteville and Decorah Formations.

Section snippets

Sample preparation

Approximately 50 g of rock were collected from thirty horizons in the Cominco SS-9 core from Millbrook Farms in Jackson County, IA (currently archived at the Iowa State Geological Survey). The sampled interval spans the upper Platteville Formation through the lower Ion Member of the Decorah Formation (Fig. 1). Samples were gently washed with methanol to remove handling and storage contamination, ground with mortar and pestle and powdered with a ball mill device. The powdered samples were Soxhlet

Lithology and paleobathymetry

The Platteville Formation is predominantly carbonate mudstone with interbedded fossiliferous packstone lenses. These lenses are similar to those observed in overlying formations, that have been attributed to tempestites (Ludvigson et al., 1996). In general, the Platteville Formation is bioturbated and contains abundant Planolites, Chondrites and Thalassinoides burrows (Byers, 1983; Dokken, 1987). CaCO3 contents range from 70 to 90% in the Platteville Formation (Table 1 and Fig. 1), and TOC

Biomarker redox indicators

Many biomarker redox proxies are affected by thermal maturity and clay contents of the mineral matrix (Peters and Moldowan, 1993). Because sampling is confined to a narrow stratigraphic interval from a single core, maturity variations are unlikely to influence biomarker proxies. However, the relative abundance of clay varies significantly in this interval (the noncarbonate fractions are composed predominantly of illite and kaolinite clays; Ludvigson et al., 1996), and some molecular

Green sulfur bacteria

Aryl isoprenoids are presumed degradation products of isorenieratene (Summons and Powell, 1987), a diaromatic carotenoid present in green sulfur bacteria (Chlorobiaceae, Liaaen-Jensen, 1978). Recent work indicates that in some depositional environments, aromatization of β-carotane can also be a significant source of aryl isoprenoids (Koopmans et al., 1996b). Compound-specific isotope analyses are therefore required to verify green sulfur bacteria as the exclusive source of aryl isoprenoids,

Redox variations

The presence of benthic macrofossils and bioturbation indicates that deposition throughout the Middle Caradocian of IA occurred under a relatively oxidizing water column. Ludvigson et al. (1996)argued that the lack of deep burrowing fabrics (i.e. Thalassinoides) in the Spechts Ferry and Guttenberg Members indicated that bottom waters were intermittently dysoxic during deposition of those units. Biomarker data do not entirely support this interpretation, and these apparently conflicting

Conclusions

We examined molecular indicators for organic-matter sources and depositional redox conditions during the Middle Caradocian from a single, continuous core from Jackson County, IA. Our molecular data and published sedimentological, ichnofossil and benthic macrofaunal data indicate oxic conditions prevailed throughout deposition of the Platteville Formation and the Ion Member of the Decorah Formation. Trace fossils are more restricted in the Spechts Ferry and Guttenburg Members of the Decorah

Note added in proof

Methylhopane identifications were confirmed by GC-MS/MS analyses performed at the Netherlands Institute for Sea Reseach.

Acknowledgements

This research was supported by a fellowship from the Earth System Science Center, Pennsylvania State University and a student grant from the Geological Society of America. We also thank Dr A. Davis and Dr G. Mitchell for assistance in petrographic analysis, Mr. D. Walizer for technical support, Dr G. Ludvigson and Dr B. Witzke at the Iowa Geological Survey for assistance in procuring samples from the SS-9 core, and Dr J. S. Sinninghe Damsté of the Netherlands Institute for Sea Research for

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