Richard D. Pancost

Consistent fractionation of 13C in nature and in the laboratory: Growth‐rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon p approximately delta e - delta p, where delta e and delta p are the delta 13C values for dissolved CO2 and for algal biomass (determined by isotopic analysis of C37 alkadienones), respectively, and if Ce is the concentration of dissolved CO2, micromole kg-1, then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, microM, and b = (25 - epsilon p)Ce. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2 = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] < or = 0.1 microM) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon p, and Ce shows that, for our entire data set, most variations in epsilon p result from variations in mu rather than Ce. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.

Controls on carbon isotope fractionation by diatoms in the Peru upwelling region

The fractionation of carbon isotopes during photosynthesis by phytoplankton is quantified for samples of suspended material collected along two transects across the Peru continental margin in 1992. The magnitude of fractionation is estimated using the δ13C of 24-methylcholesta-5,24(28)-dien-3β-ol (diatoms) and compared to that of C37.2 alkenone (haptophytes). Isotopic fractionation by diatoms exhibits a wide range and large scatter when plotted against the reciprocal of the concentration Of CO2(aq), while a strong correlation is observed for fractionation by alkenone-bearing haptophytes. Diatom growth rates, calculated from silicate concentrations and assuming Monod growth kinetics, normalized to [CO2(aq)] are well correlated to diatom fractionation factors. These results support the concept that growth rates, in addition to CO2 concentrations, impose a control on the fractionation of carbon isotopes by both taxonomic groups of algae. In addition, the very small fractionation factors for diatoms indicate that species in the Peru upwelling region employed mechanisms which actively transport inorganic carbon into cells. A size dependence is observed for the δ13C of the diatom sterol: 24-methylcholesta-5,24(28)-dien-3β-ol is enriched in 13C in samples of suspended material > 20 μm relative to the <20-μm fraction. This suggests that surface-area-to-volume ratios also impose a control on the fractionation of carbon isotopes by diatoms, a proposition that is supported by detailed cell geometry and isotopic data for two larger size fractions from one sample.

Late Middle Ordovician environmental change and extinction: Harbinger of the Late Ordovician or continuation of Cambrian patterns?

Positive excursions in carbon isotope compositions of carbonate (∼ 3‰) and organic carbon (∼ 4‰–6‰) from the late Middle Ordovician (middle Caradocian) of the midcontinent and the eastern United States indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p CO 2 , precipitating global cooling, although not necessarily ice-sheet formation. These climatic changes were associated with regional orogenic uplift, a relative rise in sea level, changes in epeiric sea circulation patterns, and carbonate platform destruction that led to regional extinction of marine benthos. The combination of sea-level rise, changing ocean circulation, and extinction in the middle Caradocian is similar to the suite of environmental changes described for Cambrian biomere boundaries, suggesting shared causes for these events. In contrast, middle Caradocian environmental changes are markedly different from the environmental patterns associated with the Late Ordovician mass extinction, despite the evidence for long-term cooling from the Middle to the Late Ordovician.

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

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.

Organic-matter source variation and the expression of a late Middle Ordovician carbon isotope excursion

The carbon isotopic composition of organic matter is a powerful tool in the study of organic carbon burial and p CO 2 through the geologic record. However, the organic-matter source can affect total organic carbon δ 13 C values and may complicate interpretation of environmental conditions. We examined the influence of organic-matter source variations on a late Middle Ordovician positive δ 13 C excursion in strata of the United States midcontinent. In a core from Iowa, the magnitude of the excursion recorded by carbonate carbon is ∼2.5‰, whereas the excursion recorded by total organic carbon is nearly 8‰. Although this difference could arise from a profound decrease in surface-water CO 2 concentrations, association of the organic-walled microfossil Gloeocapsomorpha prisca with 13 C-enriched total organic carbon (TOC) in this interval suggests that the excursion could also record a shift from normal-marine to G. prisca –dominated organic matter. Carbon isotope analyses of individual biomarkers indicate that both of these explanations are required to explain the data. Compounds inferred to be of G. prisca origin are enriched in 13 C by 7‰ relative to compounds derived from other algal sources. Furthermore, compounds exclusively from either G. prisca or other sources exhibit a positive shift of only 3.5‰. Thus, the large excursion recorded by TOC reflects, in part, a change in the proportional contribution of G. prisca to TOC. That all compounds exhibit a positive carbon isotopic shift indicates that, independent of contributions from G. prisca , a positive organic δ 13 C excursion occurred and records a change in environmental conditions such as CO 2(aq) concentrations. The 3.5‰ shift is similar in magnitude to that observed for TOC in correlative units from Pennsylvania, suggesting that this event has broad geographic significance. This work illustrates that organic-matter source variations as well as environmental conditions govern the TOC δ 13 C values and shows how compound-specific isotope analyses can resolve these multiple controls.