David J. Hollander

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.

Contrasting sulfur geochemistry and Fe/Al and Mo/Al ratios across the last oxic-to-anoxic transition in the Cariaco Basin, Venezuela

An abrupt transition from oxic to anoxic-sulfidic (euxinic) marine bottom waters occurred in the Cariaco Basin in response to increasing productivity resulting from the late Pleistocene post-glacial rise in sea level and corresponding increase in surface- water nutrient availability. The microlaminated sediments of the euxinic interval, which span the last f14.5 ky, suggest a predominance of water-column (syngenetic) pyrite formation based on (1) high pyrite sulfur (Spy) concentrations in the surficial sediment layers, (2) values for degree-of-pyritization (DOP) that generally do not increase appreciably with increasing burial, (3) ratios of total iron (FeT) to Al that are elevated above the continental baseline recorded in the underlying oxic sediments, and (4) Spy isotope trends that largely mimic the d 34 SHSof the modern water column. Intermediate DOP values in the microlaminated deposits and FeT/Al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of Fe-bearing siliciclastic accumulation. As predicted from the relative rates of siliciclastic delivery, FeT/Al and DOP data lie between end-member values observed in the modern Black Sea. As viewed broadly, FeT/Al and DOP relationships in euxinic sediments reflect the balance between syngenetic Fe scavenging and temporal and spatial gradients in siliciclastic input. Pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon (Corg). However, the upper 80 cm of the Fe-rich, Corg-poor, bioturbated sediment show evidence for a strong diffusional HSoverprint from the overlying, Fe-limited euxinic marine environment. This post-glacial transition manifests in pyrite overprints that are strongly 34 S-depleted relative to those in restricted, presently euxinic marine settings elsewhere in the world, such as the Black Sea, where the sedimentary sequence spanning the last glacial-interglacial transition begins with a shift from freshwater to Corg-poor oxic marine deposition and thus dominantly sulfate diffusion. Trends for Mo/Al ratios in the microlaminated sediments suggest that Mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits. Organic matter plays a role by enhancing HSproduction and/or by providing a substrate for Mo scavenging. Significant Mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite HS � -rich pore waters as recorded in the heavy iron sulfide overprint. We have

Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water‐column mixing, and efficient biolimiting nutrient recycling

Integrated geochemical data suggest that black shale deposition in the Devonian Geneseo Formation of western New York was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing. Once established, however, black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity. Recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced N and P regeneration from sediments, sustained high primary productivity by returning nutrients to the photic zone during mixing, and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification. This feedback was ultimately disrupted by rising siliciclastic influx, which diluted organic matter and restored benthic redox stability. The abrupt overturn of diverse, long-standing Appalachian basin marine communities may have been the result of trophic resource destabilization during Geneseo deposition.

Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction

Abstract The successful reconstruction of sea surface temperatures using alkenone paleothermometry (U37k′) has relied on the premise that there is no significant differential degradation of alkenones with different states of unsaturation during diagenetic processes. To test this assumption, we conducted a comparative study of contemporary sediments in oxic and anoxic bottom waters from the Santa Monica Basin, offshore California. Long-chain alkenones were quantified and sea surface temperature were calculated using the calibrated U37k′–T relationship of Prahl et al. (1988) . Our results show that temperature record from the oxic sediments is higher by as much as 4°C compared to those from time-equivalent anoxic sediments as a result of differential degradation of long-chain unsaturated alkenones and bioturbation mixing in the oxic sediments. The differential degradation of C37:3 vs. C37:2 alone could account for up to 2.5°C difference between these two records. This finding has significant implication in the interpretation of paleo–sea surface temperature data using alkenone paleothermometry.

Eutrophication by decoupling of the marine biogeochemical cycles of C, N, and P: A mechanism for the Late Devonian mass extinction

The Late Devonian mass extinction was unusually protracted and ecologically selective, with preferential diversity losses among reefbuilding organisms and tropical, shallow-water faunas in general. We have investigated the link between the extinction’s unique characteristics and changes in biogeochemical cycling through analyses of the δ 13 C and C:N:P atomic ratios of organic matter buried across the Kellwasser Horizons in western New York State. Each horizon is characterized by (1) a long-term, + 4‰–5‰ excursion in δ 13 C, ~3‰ of which occurs within the horizon, and (2) a dramatic increase in the burial ratios of C:N:P, from values of ~100:15:1 to an average of ~5000:170:1. On the basis of these results, we propose that (1) increased efficiency of biolimiting nutrient recycling, resulting from cyclic water column stratification and mixing, promoted eutrophication during Kellwasser deposition in New York, and (2) the isotope excursions represent the composite effect of long-term, global organic C burial, and local changes in photosynthetic C isotope fractionation related to nutrient availability. This eutrophication model forges a mechanistic link between proposed Late Devonian climatic cooling and the selective demise of taxa likely to have been narrowly adapted to oligotrophic conditions.

Microbially mediated carbon cycling as a control on the δ13C of sedimentary carbon in eutrophic Lake Mendota (USA): new models for interpreting isotopic excursions in the sedimentary record

An isotopic study of various carbon phases in eutrophic Lake Mendota (Wisconsin, USA) indicates that the δ13C composition of sedimentary organic and inorganic carbon has become more negative in response to increasing microbially mediated carbon cycling and processes associated with the intensification of seasonal and long-term eutrophication. Progressive increases in the contributions of isotopically depleted chemoautotrophic and methanotrophic biomass (reflected in the −40 to −90‰ values of hopanols and FAMES), attributed to seasonal and long-term increases in production and expansion of the anaerobic water mass, accounts for carbon isotopic trends towards depleted δ13C values observed in both seasonal varves and over the past 100 years. Changes in the intensities of certain microbial processes are also evident in the sedimentary geochemical record. During the period of most intense cultural eutrophication, when the oxic-anoxic interface was located close to the surface, methanogenesis/methanotrophy and the oxidation of biogenic methane increased to the extent that significant quantities of 13C-depleted CO2 were added into the epilimnion. This depleted CO2 was subsequently utilized by phytoplankton and incorporated into CaCO3 during biogenically induced calcite precipitation. A comparative study between eutrophic Lakes Mendota and Greifen, further indicate that the extent of nutrient loading in the epilimnion determines whether the δ13C record of sedimentary organic carbon reflects intensification of microbial processes in the hypolimnion and sediments, or changes in the primary productivity in the photic zone. From this comparison, a series of eutrophication models are developed to describe progressive transitions through thresholds of microbial and eukaryotic productivity and their influence on the δ13C record of sedimentary carbon. With increasing eutrophication, the models initially predict a negative and then a subsequent positive carbon isotopic excursion reflecting the changing influence of 13C-deleted microbial biomass relative to 13C-enriched photoautrophic biomass. These eutrophication models provide a framework to evaluate carbon cycling processes in modern environments and have significant implications for interpreting carbon isotopic excursions in the sedimentary record.

Reduced sulfur in euxinic sediments of the Cariaco Basin: sulfur isotope constraints on organic sulfur formation

Reduced sulfur accumulation in Holocene and latest Pleistocene euxinic marine sediments from the Cariaco Basin, Venezuela, was investigated to constrain the timing and possible pathways of organic matter (OM) sulfurization. Data were collected for a diverse suite of sulfur species, including concentrations and sulfur isotope compositions of pore-water sulfide, pore-water sulfate, pyrite sulfur, total organic sulfur (TOS), kerogen sulfur (KS), and polar bitumen sulfur (PBS). Results suggest that there was a period during which almost no diagenetic pyrite formed in the sediments of the Cariaco, coincident with a shift from high to lower sedimentation rates and a concomitant change in the delivery of organic matter to the sediments. The sulfur isotope composition of organic matter was predicted based on assumed pathways using weighted isotopic mass balance calculations and compared to measured isotope values for organic sulfur. These results indicate that organic sulfur is derived primarily from pore-water sulfide, with minor contributions from primary bio-sulfur (e.g., in proteins derived from algae and bacteria). The predicted sulfur isotope values of organic sulfur compounds (OSC) suggest that pore-water sulfide is the ultimate source of reduced sulfur for incorporation into organic matter. It is possible, however, that reactive sulfur intermediates such as elemental sulfur or polysulfides react directly with organic matter. These intermediate sulfur species are likely formed through partial oxidation of sulfide by anaerobic sulfide-oxidizing microbes living in the sediments.

Differential contribution of bacteria to sedimentary organic matter in oxic and anoxic environments, Santa Monica Basin, California

Two sediment box-cores in the Santa Monica Basin (SMB), one from the depocenter (900 m water depth) with anoxic bottom water and one from the periphery (840 m water depth) with oxic bottom water, were comparatively studied to assess the role of bottom water oxygen conditions to organic matter preservation and the differential contribution of bacteria to sedimentary organic matter accumulation and composition. Fatty acids from the sediment extracts were analyzed for their abundance and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain FAs from the SMB sediment extracts are sourced by bacteria as a result of intense microbial recycling and resyntheses of organic matter, especially in the aerobic environment at the periphery site. Long-chain fatty acids are probably derived from terrestrial sources and partially sourced by microbial processes. Very depleted δ13C values (−30 to −45‰) of bishomohopanoic acids and triglycerides suggest the presence of chemoautotrophic bacteria in the anoxic water column and in the sediments. There is a much greater contribution from anaerobic microbial biomass and chemoautotrophic bacteria in the depocenter sediments relative to the periphery sediments. In addition, organic matter exhibits bulk and molecular characteristics of enhanced preservation under the condition of anaerobic decomposition at the depocenter site, especially at the sediment/water interface and the first couple of centimeters. Together, enhanced preservation with the addition of microbial biomass could explain the higher organic accumulation rates and H/C ratios in the basin depocenter and the periphery sediments and the trend of decreasing δ13C values of bulk organic matter in the depocenter sediments. This study has implications for understanding the role of anoxia and associated bacterial processes in controlling the geochemical characteristics ancient organic carbon-rich sediments deposited under oxic and anoxic conditions.

Climate‐induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela

A high-resolution molecular organic geochemical study of sediments in the anoxic Cariaco Basin indicates significant changes in primary productivity and planktonic community structure associated with the transition from the Younger Dryas to the Holocene. Variations in climate conditions over the past 12 14C kyr have induced large-scale changes in upwelling intensity, which directly affected levels of primary productivity as reflected in accumulation rates of bulk productivity proxies. Concentrations and accumulation rates of sterol and alkenone biomarkers have been used to identify how productivity changes affected the structure of the planktonic ecosystem. A shift in the dominant primary producer from diatoms (Younger Dryas) to coccolithophores (Holocene) is identified. If productivity and ecosystem variations like those identified in the tropical upwelling zone of the Cariaco Basin region, occur throughout the tropical oceans, they have the potential to affect global climate through perturbations in the biogeochemical cycle of carbon.

Early diagenesis of bacteriohopanepolyol derivatives: Formation of fossil homohopanoids

Abstract Diagenetic pathways of bacteriohopanepolyol derivatives are proposed based on the concentrations and 13C contents of homohopanes, homohop-17(21)-enes, benzohopanes, hopanoid thiophenes and sulphides, and macromolecularly S-bound homohopanes present in the extracts of twelve composite one metre samples from a 120 m core recovered from the Upper Cretaceous Jurf ed Darawish Oil Shale (Jordan). A large part (>80–95%) of the pentakishomohopane skeleton occurs in a S-bound form. This reveals the selective preservation of the C35 hopane skeleton by sulphur sequestration and provides a theoretical basis for the homohopane index as an indicator of anoxia in past depositional environments. A smaller part (>50–80%) of the total extended hopane skeletons (C31–C35) occurs in a S-bound form. Of the non-sulphur-containing hopanoids the homohop-17(21)-enes dominate. These latter components show a gradual increase of 22S epimers with depth (45–52%) towards the thermodynamic equilibrium (52–53%) as calculated by molecular mechanics. Molecular mechanic calculations indicate that this increase can be explained by either isomerisation of 22R hop-17(21)-enes or by isomerisation of double bonds of homohopenes formed by dehydration of bacteriohopanepolyols “en passant” isomerising the chiral centre at C-22. A combination of these two pathways is also possible and provides an explanation for different δ13C values of pairs of 22R and 22S epimers. Isomerisation of 17β,21β(H)-homohopane to 17α,21,β(H)-homohopane carbon skeletons occurs for all compound classes in a very narrow depth span (ca. 20 m) and is probably induced by small differences in thermal history. Compound-specific carbon isotope analyses indicated that the series of homohop-17(21)-enes have in some cases significant differences in 13C content, indicating that at least two different sources have contributed to this series of components. Differences with macromolecularly S-bound C35 hopane skeletons and free C31 hopanes 13C contents are in some cases even larger. These data show that the diagenetic pathways of bacteriohopanepolyol derivatives are more complex than previously recognized and reveal that multiple precursor bacteriohopanepolyol derivatives prone to different diagenetic pathways have to be envisaged to account for the differences observed.