Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial ...transition. The mechanisms driving this hypoxia remain under debate. We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4-5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals, and enhanced (15)N/(14)N ratio of organic matter, collectively suggest association with high export production. A decrease in (18)O/(16)O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.
The present paper is the result of a workshop sponsored by the DFG Research Center/Cluster of Excellence MARUM "The Ocean in the Earth System", the International Graduate College EUROPROX, and the ...Alfred Wegener Institute for Polar and Marine Research. The workshop brought together specialists on organic matter degradation and on proxy-based environmental reconstruction. The paper deals with the main theme of the workshop, understanding the impact of selective degradation/preservation of organic matter (OM) in marine sediments on the interpretation of the fossil record. Special attention is paid to (A) the influence of the molecular composition of OM in relation to the biological and physical depositional environment, including new methods for determining complex organic biomolecules, (B) the impact of selective OM preservation on the interpretation of proxies for marine palaeoceanographic and palaeoclimatic reconstruction, and (C) past marine productivity and selective preservation in sediments. It appears that most of the factors influencing OM preservation have been identified, but many of the mechanisms by which they operate are partly, or even fragmentarily, understood. Some factors have not even been taken carefully into consideration. This incomplete understanding of OM breakdown hampers proper assessment of the present and past carbon cycle as well as the interpretation of OM based proxies and proxies affected by OM breakdown. To arrive at better proxy-based reconstructions "deformation functions" are needed, taking into account the transport and diagenesis-related molecular and atomic modifications following proxy formation. Some emerging proxies for OM degradation may shed light on such deformation functions. The use of palynomorph concentrations and selective changes in assemblage composition as models for production and preservation of OM may correct for bias due to selective degradation. Such quantitative assessment of OM degradation may lead to more accurate reconstruction of past productivity and bottom water oxygenation. Given the cost and effort associated with programs to recover sediment cores for paleoclimatological studies, as well as with generating proxy records, it would seem wise to develop a detailed sedimentological and diagenetic context for interpretation of these records. With respect to the latter, parallel acquisition of data that inform on the fidelity of the proxy signatures and reveal potential diagenetic biases would be of clear value.
Sorption of organic matter to mineral surfaces in marine sediments stabilizes the component molecules, slowing remineralization rates by up to five orders of magnitude. Sorptive protection of labile ...organic matter is discussed.
Total organic carbon (TOC) and biogenic silica (opal) content, elemental (C/N) and isotopic (δ
13C, δ
15N) composition of organic matter and the content of lipid biomarkers derived from both marine ...and terrestrial sources constrain relative contributions from marine productivity and continental erosion to surface sediments throughout coastal SE Alaska (54°N to 61°N). TOC and opal content are very high (up to 8% and 33% by weight, respectively) in fjords and inlets south of Icy Strait (∼58°N) and uniformly low at offshore sites to the south, and at both offshore and inland sites to the north (averaging 0.6±0.3% and 2.3±1.8%, respectively). TOC and opal mass accumulation rates (MARs, based on bulk density and
210Pb-derived sediment MAR) suggest dilution with terrigenous, inorganic detrital materials accounts for the low concentrations of both biogenic phases in sediments from the glacial tidewater fjords of Muir and Yakutat Bays but not elsewhere. C/N, δ
13C, and δ
15N indicate a dominant marine origin for organic matter deposited at most sites. This conclusion implicates elevated primary productivity in inland waters to the south with diatoms, based on opal results, being the dominant contributor. A very significant terrestrial organic fraction (25–50%) is contained in sediments deposited on the continental shelf to the north of 58°N. Hydrocarbon biomarkers indicate the terrestrial fraction in sediments from this region is represented by old organic matter (kerogen) likely contained within riverborne particles eroding from now heavily glaciated adjacent landscapes. In sediment to the south, the terrestrial fraction is traced to modern soil organic matter eroded from the now non-glaciated, heavily forested adjacent landscape. Our study provides a framework to guide future investigations of short- (anthropogenic) to long- (Holocene) term environmental and/or climate change in this region through down-core, stratigraphic analysis.
Recent works have investigated use of the hydrogen isotopic composition of C37 alkenones (δDK37s, lipid biomarkers of certain haptophyte microalgae, as an independent paleosalinity proxy. We discuss ...herein the factors impeding the success of such an application and identify the potential alternative use of δDK37s measurements as a proxy for non-thermal, physiological stress impacts on the U37K' paleotemperature index. Batch-culture experiments with the haptophyte Emiliania huxleyi (CCMP 1742) were conducted to determine the magnitude and variability of the isotopic contrasts between individual C37 alkenones. Further experiments were conducted with Emiliania huxleyi (CCMP 1742) andGephyrocapsa oceanica (PZ3-1) to determine whether, and to what extent, δDK37s varies between the physiological extremes of nutrient-replete exponential growth and nutrient-depleted senescence. Emiliania huxleyi was observed to exhibit an isotopic contrast between di- and tri-unsaturated C37 alkenones (αK37:3-K37:2≈0.97) that is nearly identical to that reported recently by others for environmental samples. Furthermore, this contrast appears to be constant with growth stage. The consistency of the offset across different growth stages suggests that a single, well-defined value for αK37:3-K37:2 may exist and that its use in an isotope mass-balance will allow accurate determination of δD values for individual alkenones without having to rely on time- and labor-intensive chemical separations. The isotopic fractionation between growth medium and C37 alkenones was observed to increase dramatically upon the onset of nutrient-depletion-induced senescence, suggesting that δDK37s may serve as an objective tool for recognizing and potentially correcting, at least semi-quantitatively, for the effects of nutrient stress on U37K' temperature records.
Profiles of alkenone concentration, the abundance of the calcified alkenone-producing species
Emiliania huxleyi and
Gephyrocapsa oceanica as well as nutrient and phytoplankton pigment concentrations ...were measured through the euphotic zone in surface waters of the Gulf of California during cruises in two different summers (2004, 2005) and one winter (2005). As determined using a biomarker and pigment inventory approach,
E. huxleyi and
G. oceanica, always the most abundant coccolithophore species at all sampling sites and seasons, represented only a minor fraction of total haptophytes which themselves constituted only a minor fraction of the total phytoplankton community. In winter, alkenone concentration was highest at the surface and displayed relatively uniform values with depth throughout the nutrient-replete euphotic zone. In summer, it display a subsurface peak in nitrate-depleted waters situated just above the nitracline and the deep chlorophyll maximum layer (DCML), a feature found to be well-developed at most sites. Maximum biomarker abundance was comparable in the two seasons, although the putative alkenone-producing coccolithophore species were more abundant in winter than in summer. Estimates of cellular alkenone content for the combined abundance of
E. huxleyi and
G. oceanica at each sampling depth showed that cells residing in the nitrate-depleted upper euphotic zone in summer were typically more enriched than those residing in nutrient-replete waters found deeper in the euphotic zone or throughout the euphotic zone in winter. The former estimates were very similar to values documented in laboratory cultures of
E. huxleyi harvested in a nutrient-depleted stationary growth phase and the latter to those harvested in an exponential growth phase, under the influence of incipient light limitation. Overall, our findings imply that calcifying forms of
E. huxleyi and
G. oceanica can account for the measured alkenone concentration at all sites and sampling times. Although the life history of these organisms is known to include non-calcifying stages, it is unnecessary to invoke such an explanation as the cause for the elevated alkenone content of cells from nutrient-depleted surface waters in summer.
Examining fluxes of biogeochemical constituents at the mouth of an estuary is necessary for assessing the modification of terrigenous-source materials in the estuary prior to reaching the ocean. In ...many rivers and estuaries, including the Columbia River estuary (CRE), methane is highly enriched with respect to oceanic concentrations and the equilibrium solubility of the atmospheric gas. We developed a methane budget for the CRE to examine the potential for significant modification of the estuarine methane budget by lateral exchange with peripheral tide flats. We accomplished the challenging task of constraining the net transfer through the estuary-ocean interface using novel instrumentation: a rapid methane analyzer combined with a membrane-contactor interfaced with a pumped-sampling undulating towed vehicle. Transport of riverine methane into the CRE was essentially balanced by losses due to flux to the atmosphere (42%), microbial oxidation in the water column (21%), and transport to the ocean (32%), suggesting limited net effect of lateral tide flat processes on the CRE methane budget. Estimated uncertainty bounds constrained lateral sink/source terms within −30% to +20% of the primary river input. This result contrasts with a number of prior studies of methane cycling in estuaries that reported dominant contributions from lateral sources and relatively minor export to the coastal ocean. The magnitude of lateral supply of methane is a useful indicator of the hydrologic source potential of other related signals of organic matter remineralization from anoxic or suboxic settings in the estuary.
This study evaluated seasonal variability in the quantity and quality of particulate organic matter exported by the Columbia River into its estuary. Samples of suspended particulate material (SPM), ...collected monthly over a 1 year period (November 1995–October 1996) from a freshwater site in the Columbia River, near the head of its estuary, were analyzed for total aluminum, particulate organic carbon and nitrogen (POC and PN, respectively) and algal pigments (chlorophyll and carotenoids). High Al content, averaging 7.9% by weight throughout the year, indicated that detrital minerals accounted for the majority of SPM mass at all times. Organic matter (approximately 2 × POC) varied on a seasonal basis both in terms of its mass contribution to SPM (5–26% by weight) and its chemical composition. In winter, organic matter originated mainly from allochthonous sources, most likely from erosion of soils. Diatoms, the major phytoplankton group present as inferred from pigment analysis, “bloomed” in April–June 1996, resulting in increased autochthonous contribution to POC during spring and summer as well as increased organic matter content of SPM introduced to the estuary. Comparison with results for the same site in the mainstem Columbia obtained between 1990 and 1999 as part of a land-margin ecosystem research project shows that enhanced algal production in springtime is not a feature unique to our data set, at least in the Columbia River’s present highly regulated state of hydrographic operation. The possibility is discussed that recent human influence, principally dam construction, has significantly altered the historical SPM concentration and the chemical composition of its associated organic matter.
Three cohorts of Pacific oyster (Crassostrea gigas) larvae at Whiskey Creek Shellfish Hatchery (WCH) in Netarts Bay, Oregon, were monitored for stable isotope incorporation and biochemical ...composition: one in May 2011 and two in August 2011. Along with measures of growth and calcification, we present measurements of stable isotopes of carbon in water, algal food, and the shell and tissue, and nitrogen in food and tissue across larval development and growth. These relatively unique measures through larval ontogeny allow us to document isotopic shifts associated with initiation and rate of feeding, and the catabolism of C-rich (lipid) and N-rich (protein) pools. Similar ontological patterns in growth and bulk composition among the cohorts reinforce prior results, suggesting that the creation of the initial shell is energetically expensive, that the major carbon source is ambient dissolved inorganic carbon, and that the major energetic source during this period is maternally derived egg lipids. The May cohort did not isotopically reflect its food source as rapidly as the August cohorts, indicating slower feeding and/or higher catabolism versus anabolism. Our measurements also document differences in bulk turnover of organic carbon and nitrogen pools within the larvae, showing far greater conservation of nitrogen than carbon. These stable isotope and bulk biochemical measurements appear to be more sensitive indicators of sub-lethal environmental stress than the commonly used metrics of development and growth.
•This study reviews the effect of degradation on alkenones.•Grazing, sulfurization and anaerobic biodegradation do not affect the U37K′ index.•Autoxidation, aerobic biodegradation and stereomutation ...may, however, alter it.•Users of the U37K′ proxy should therefore be aware of its possible degradative alteration.
Lipid biomarkers in sediments are widely used to infer environmental conditions occurring in the geological past, but such reconstructions require careful consideration of the biotic and abiotic processes that degrade and alter lipid biomarker compositions before and after deposition. In this study, we use alkenones produced by haptophyte microalgae to explore the range of effects of these degradative processes. Alkenones are now perhaps the best studied of all biomarkers, with several hundred references on their occurrence in organisms, seawater and sediments. Much information has been obtained on their degradation from laboratory incubation studies and inferences from changes in their distribution in aquatic environments. Although alkenones are often considered as more stable than many other lipid classes, it is now clear that their distributions can be affected by processes such as prolonged oxygen exposure, aerobic bacterial degradation and thiyl radical-induced stereomutation which, in some cases, can lead to changes in the proportions of the alkenones used in the U37K′ temperature proxy. The same set of chemical and biological processes act on all lipids in aquatic environments and, in cases where there is a marked difference in reactivity, this may lead to significant changes in the biomarker distributions and relative proportions of different lipid classes.
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