Hydrogen isotopic composition of C₃₇ alkenones and the ecology and physiology of coccolithophores

A three-part study was conducted into the impact of physiological and ecological variables on the net isotopic fractionation of hydrogen, α[subscript K₃₇], expressed in C₃₇ alkenones. First, alkenone-producer production, abundance, and export were characterized in the summertime Gulf of California and Eastern Tropical North Pacific using compound-specific, labeled in situ incubations. It was found that these organisms routinely exhibited maximum production rates at depths associated with subsurface chlorophyll features and the nitracline, as opposed to the N-depleted surface, and that the relative contribution of coccolithophore productivity to overall productivity was decoupled from nutrient conditions. Our observations suggest that, in sufficiently well-stratified settings, a coccolithophore-favorable 'mid-to-low nutrient' niche may be absent. Second, results from a culture study were compared to samples from the initial field campaign in order to more firmly establish the physiological controls on the δD of alkenones. Nutrient-limitation experiments in culture, combined with previously published data, show that net fractionation between the growth medium and alkenones (α[subscript K₃₇]) increases rapidly with increasing cellular alkenone content and production rate, and, by extension, growth phase. To explain these results, a mechanism is proposed in which changing NADPH sources result in isotopically-depleted lipids at both high growth rates and in stationary phase growth. Comparison of these results to the field samples suggests that, in the water column, this dynamic (a relationship α[subscript K₃₇] and growth rate) drives correlations between α[subscript K₃₇] and both cell abundance and the carbon-fractionation term εp. Lastly, the chemical and isotopic composition of alkenones was measured in sediment samples along a transect of the North-American Pacific margin from ~42°N to the tip of Baja California. It was found that both core-top and LGM intervals expressed a strong relationship between the temperature proxy U₃₇[superscript K'] and estimated α[subscript K₃₇]. If covariation between U₃₇[superscript K'] and estimated α[subscript K₃₇] is robust in settings such as these, then paired analysis of U₃₇[superscript K'] and alkenone δD may enable paleoceanographic estimates water δD. In light of the results of the second study, it is proposed that this relationship derives from a control of temperature on cellular division rate on sedimentary time scales. Deviations from this relationship were tentatively interpreted as indicative of export from locations where alkenone producers are/are not subjected to nutrient deprivation before sedimentation. Overall, the results of this three-part study suggest A) that coccolithophore-derived inorganic carbon export may serve as a weaker positive feedback on atmospheric CO₂, in the future, than previously suggested; B) that α[subscript K₃₇] may lend valuable context to studies of alkenone-producer ecology, and C) that alkenone δD may prove a better than expected hydrologic proxy in marine settings and/or a coccolithophore-specific growth rate proxy.Keywords: deuterium, Baja, coccolithophore, LGM, lipids, fractionation, stratification, alkenon

North Pacific deglacial hypoxic events linked to abrupt ocean warming

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 ¹⁵N/¹⁴N ratio of organic matter, collectively suggest association with high export production. A decrease in ¹⁸O/¹⁶O 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

Rainshadow effect on hydrogen isotopes in leaf wax n-Alkanes across the Cascade Mountains of Washington, USA

International audienceHydrogen isotope ratios are sensitive tracers of the water cycle with the potential to constrain the timing of mountain building episodes in the geologic past. This study presents hydrogen isotope ratios (2H/1H or δ2H) of n-alkanes derived from the leaf wax of terrestrial plants, collected from trees and soils along an east–west transect across the Cascade mountain range of Washington State, USA. Along this transect, the δ2H values of n-alkanes (δ2Halkane) in surface soils and gymnosperm leaves are well-correlated with the mean annual δ2H of precipitation (δ2Hprecip) derived from instrument-model reanalysis products (R2 = 0.346 to 0.558, p < 0.001 for soil alkanes, R2 = 0.667 to 0.844, p < 0.001 for gymnosperm alkanes) and with measured δ2H values of nearby surface waters (R2 = 0.451, p < 0.001). All data from this study show increasing deuterium (2H) depletion with distance from the coast in response to Rayleigh distillation induced by the Cascade Mountains. Assessed within the context of a global dataset of 235 published soil δ2Halkane values vs δ2Hprecip, the Cascades results are statistically indistinguishable. However, the gymnosperm leaf δ2Halkane vs δ2Hprecip data from the Cascades differ from a global set of 87 such sites, with a steeper relationship. This is attributed to regional differences between the Cascades and other locations that include higher latitude, higher altitude, and the presence of a strong, narrow climatic gradient. The strong imprint of the Cascade rainshadow on δ2Halkane validates its use as a proxy for the timing of uplift of this mountain range when measured in suitable sedimentary rocks

Alkenone δD as an ecological indicator: A culture and field study of physiologically-controlled chemical and hydrogen-isotopic variation in C37 alkenones

A combined culture and field study was conducted in order to (1) more firmly identify the physiological controls on hydrogen isotopic composition of C37 alkenones produced by open-ocean coccolithophores and (2) determine the degree to which these controls are manifested in a natural water column. Nutrient-limitation experiments in culture, combined with previously published data, show that net fractionation between the growth medium and alkenones (αK37) varies with cellular alkenone content and production rate, and, by extension, growth phase. It is hypothesized that the relationship of αK37 with cellular alkenone content and production rate is due to increased use of anabolic NADPH in response to high rates of lipid synthesis. Euphotic zone profiles of δDK37, measured in suspended material from the Gulf of California and Eastern Tropical North Pacific, decreased with depth and light availability, and did not correlate in any expected way with previously-suggested controls on αK37. It is possible that the field data are driven by behavior in light-limited cells that is not represented by the available, nutrient-limited culture data. If true, δDK37 may have utility as an indicator of production depth in settings prone to subsurface production maxima. Relationships between αK37, cell density, and the carbon-isotopic fractionation term εp, however, suggest that αK37 acts an indicator of growth rate, which in this setting is only partially dependent on light, consistent with our interpretation of the culture data. If this latter interpretation proves correct, δDK37 may be a powerful ecological proxy specific to these climatically-important, calcifying, temperature-encoding species

An Arctic Ocean paleosalinity proxy from d2H of palmitic acid provides evidence for deglacial Mackenzie River flood events

The hydrogen isotopic composition (2H/1H, or d2H) of palmitic acid (PA) was measured in surface sediments from the Laptev and Kara Seas in the Russian Arctic to evaluate its use as a paleohydrographic proxy. d2HPA values in surface sediments varied by 118‰ over a 21 ppt range in mean annual surface salinity, and the two properties were highly correlated (R2 ¼ 0.8, p < 0.001) according to the relationship d2HPA ¼ 4.22 (±0.60)*S - 338 (±15). In contrast, d2H values of vascular plant wax n-alkanes (nC27, nC29, nC31) did not change systematically with salinity. These differing lipid d2H trends support the interpretation of PA as derived primarily from marine microalgae at these sites. Both the range and absolute values of d2HPA compared favorably to those predicted from published Arctic Ocean salinity and water isotope data and the expected response of d2HPA to salinity in cultured phytoplankton. Some 64e74% of the observed sedimentary d2HPA increase is estimated to have resulted from increasing d2Hwater values, with the remainder resulting from decreased 2H-discrimination during lipid biosynthesis at higher salinities. The large signal and high sensitivity of d2HPA to surface salinity changes in the Russian Arctic was exploited to test the hypothesis that floodwaters emanated from the Mackenzie River during the late deglacial period. Measurements of d2HPA were performed in a sediment core from the continental slope off the Mackenzie River in the Canadian Arctic. In samples from the top Bølling/Allerød-Younger Dryas period, reconstructed surface salinities (and d2HPA values) off the Mackenzie River declined from 20 ("253‰) to 16 ("269‰) before rebounding to 24 ("236‰) in the early Holocene, close to the modern value of ~25. A large salinity depression in the Canadian Arctic just prior to the start of the Younger Dryas would support the hypothesis of a northern routing of flood-waters from glacial Lake Agassiz via the Mackenzie River as a trigger for the Younger Dryas event

Alkenone δD as an ecological indicator: A culture and field study of physiologically-controlled chemical and hydrogen-isotopic variation in C37 alkenones

A combined culture and field study was conducted in order to (1) more firmly identify the physiological controls on hydrogen isotopic composition of C37 alkenones produced by open-ocean coccolithophores and (2) determine the degree to which these controls are manifested in a natural water column. Nutrient-limitation experiments in culture, combined with previously published data, show that net fractionation between the growth medium and alkenones (αK37) varies with cellular alkenone content and production rate, and, by extension, growth phase. It is hypothesized that the relationship of αK37 with cellular alkenone content and production rate is due to increased use of anabolic NADPH in response to high rates of lipid synthesis. Euphotic zone profiles of δDK37, measured in suspended material from the Gulf of California and Eastern Tropical North Pacific, decreased with depth and light availability, and did not correlate in any expected way with previously-suggested controls on αK37. It is possible that the field data are driven by behavior in light-limited cells that is not represented by the available, nutrient-limited culture data. If true, δDK37 may have utility as an indicator of production depth in settings prone to subsurface production maxima. Relationships between αK37, cell density, and the carbon-isotopic fractionation term εp, however, suggest that αK37 acts an indicator of growth rate, which in this setting is only partially dependent on light, consistent with our interpretation of the culture data. If this latter interpretation proves correct, δDK37 may be a powerful ecological proxy specific to these climatically-important, calcifying, temperature-encoding species