Josef P. Werne

A tale of shales: the relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian basin

Abstract A new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged. More firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry, this consensus recognizes the interdependent roles of sedimentation, primary production, and microbial metabolism in favor of earlier end-member models (e.g., “production vs. preservation”). In this study, one of the classic stratigraphic sequences upon which the “preservation” end-member was based is re-interpreted in light of this new consensus. The study employs an extensive new sedimentological–biogeochemical database from cores drilled in western New York. The database spans over 500 m and 15 my of Devonian deposition in the Appalachian basin and provides a framework for comparative study of organic matter burial. The major conclusions are: (1) few organic-rich units were deposited under pervasive anoxic–sulfidic water columns; (2) establishment and breakdown of seasonal thermoclines, on annual or longer timescales, were the predominant mode of stratification; and (3) under such conditions, remineralization of bio-limiting nutrients may have played a key role in organic matter burial by creating a “eutrophication pump.” This pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments, and episodically in bottom waters. A final conclusion asserts that the master variable for organic matter accumulation was relative sea-level change, which exerted influence on clastic dilution, preservation, and production processes. Sea-level rise events led to sediment starvation and organic carbon concentration in distal basin sediments, as well as to decreased effectiveness of seasonal mixing and thus longer build-up intervals for remineralized nutrients. Episodic mixing of nutrient-enriched bottom waters led to enhanced production. Ultimately, increased clastic sediment delivery and water column mixing during relative sea-level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply, thus terminating deposition of strata enriched in organic carbon.

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

Crenarchaeotal membrane lipids in lake sediments: A new paleotemperature proxy for continental paleoclimate reconstruction?

Paleoclimate studies of continental environments have been hampered by the lack of an independent paleotemperature proxy. A novel sea-surface temperature proxy has been proposed for marine systems based on membrane lipids of marine crenarchaeota. This proxy will provide an independent continental paleotemperature tool that will allow paleoclimatologists to address fundamental questions about temperature variability in continental environments and its relationship to climate change.

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.

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.

Extended megadroughts in the southwestern United States during Pleistocene interglacials

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

Wet and arid phases in the southeast African tropics since the Last Glacial Maximum

Plant leaf wax carbon isotopes provide a record of C 3 versus C 4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and an antiphase relationship prevailed. The abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the Intertropical Convergence Zone (ITCZ) during the last glacial. Southward displacements of the ITCZ are also linked to arid conditions in southeast Africa during the Younger Dryas and the Little Ice Age.

Timing of early diagenetic sulfurization of organic matter: A precursor-product relationship in Holocene sediments of the anoxic Cariaco Basin, Venezuela

Abstract The incorporation of reduced inorganic sulfur into organic matter is a significant mechanism for the preservation of functionalized organic compounds in the sedimentary environment, but the timing of this process is not currently known. Analysis of organic matter in the Holocene and latest Pleistocene sediments of the Cariaco Basin indicates conversion of a triunsaturated tricyclic triterpenoid, (17E)-13β(H)-malabarica-14(27),17,21-triene, identified by isolation and subsequent one- and two-dimensional 1 H and 13 C NMR spectroscopy, to a monounsaturated triterpenoid thiane through incorporation of reduced inorganic sulfur species into the isoprenoid side chain within the upper 6 m of sediments. Time control provided by AMS 14 C dates was used in conjunction with measured relative abundances of the two compounds to calculate the reaction rate for the sulfurization of this triterpenoid, and empirically determine the first order rate constant (2 × 10 −4 yr −1 ) for the reaction. This is the first time that such a precursor-product relationship has been positively identified for a sulfurization reaction in sediments.

An interlaboratory study of TEX86 and BIT analysis of sediments, extracts, and standard mixtures

Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX86 values (reproducibility +/- 3-4 degrees C when translated to temperature) but a large spread in BIT measurements (reproducibility +/- 0.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX86 values ranged from 1.3 to 3.0 degrees C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the true (i.e., molar-based) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

A progressively wetter climate in southern East Africa over the past 1.3 million years

African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial–interglacial change intensified worldwide. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°–14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals.