TRACING TIME IN THE OCEAN: UNRAVELING DEPOSITIONAL AND PRESERVATIONAL TIMESCALES USING COMPOUND-SPECIFIC RADIOCARBON ANALYSIS OF BIOMARKERS FROM MARINE SEDIMENTS


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Gesine.Mollenhauer [ at ] awi.de

Abstract

Carbon cycle dynamics between the different inorganic and organic carbon pools play an important role in controlling the atmospheric chemical composition, thus, regulating the Earth’s climate. Atmospheric CO2 is fixed into biomass by photosynthesis of terrestrial and marine primary producers. Until final burial in marine sediments, the biologically fixed carbon that escapes remineralisation undergoes exchange between various active carbon reservoirs. Until now, the timescales on which terrestrial and marine organic matter is exchanged between the terrigenous, oceanic, and sedimentary carbon pool as well as the residence time in the respective reservoirs are still poorly understood. Compound-specific radiocarbon analysis of biomarkers provides a powerful tool to determine the temporal scales of such exchange processes. Here, the timescales of different physicochemical and sedimentological processes are resolved including timescales of lateral sediment transport within the ocean, terrestrial residence times of terrigenous organic matter prior to delivery to the ocean, and timescales of organic matter preservation in the ocean. Radiocarbon ages of co-occurring alkenones, foraminiferal tests, and total organic carbon (TOC) were measured to estimate the timescales of lateral sediment transport within the Panama Basin. Such transport processes are likely to occur at the sedimentwater interface prior to initial sedimentation or following resuspension and can lead to prolonged residence times of organic matter in the water column before final burial in the sediments. Accordingly, this process may cause age offsets between organic matter easily prone to resuspension (alkenones) and denser sediment constituents (foraminifera). In Panama Basin late glacial to Holocene sediments from cores ME0005- 24JC, Y69-71P, and MC16 all sediment constituents mostly agree well in age, indicating no significant addition of pre-aged alkenones or contribution of aged terrigenous organic matter. Therefore, lateral sediment transport is not inferred. However, evidence from previous studies indicates strong sediment focusing at the investigated core locations. Thus, if lateral transport occurs, radiocarbon ages of alkenones, TOC, and foraminifera indicate rapid, syndepositional (i.e. within decades), and local nepheloid layer transport rather than remobilization of aged sediments. Transport within only a few decades inhibits temporal decoupling of proxies from different grain size fractions, thus, validating multi-proxy paleoceanographic reconstructions in the study area. Nevertheless, anomalously old foraminiferal tests were found in one glacial depth interval of core Y69- 71P resulting from downslope transport along the northern Carnegie Ridge. This process might bias paleoceanographic reconstructions for core Y69-71P based on foraminifera. Radiocarbon ages of long-chain vascular plant n-alkanes and n-fatty acids, and TOC were determined to estimate their average terrestrial residence time prior to burial ABSTRACT II in Black Sea sediments and the underlying controlling factors. Storage of terrigenous organic matter in terrestrial reservoirs, such as soils, is likely to produce age offsets between marine and terrigenous organic matter in marine sediments, which is critical when continental climate is reconstructed. Average terrestrial residence times in different river drainage areas of the Black Sea deduced from n-C29+31 alkanes and n-C28+30 fatty acids of river mouth stations range from 900±70 years to 4400±170 years. River catchment size is the major morphological control on terrestrial residence time in climatically similar drainage areas. A climatic controlling factor cannot unambiguously be determined, but Mediterranean climate appears to increase continental carbon turnover compared to continental climate. Along-transect data imply petrogenic n-C29+31 alkanes contribute to the vascular plant n-C29+31 alkanes. As a result, n-C28+30 fatty acids provide better estimates of average terrestrial residence time. Along-transect data furthermore reveal that n-C29+31 alkanes as well as n-C28+30 fatty acids are supplied by both riverine (nearshore) and aeolian (offshore) transport mechanisms. Interestingly, aeolian vascular plant biomarkers are pre-aged as well although to a lesser extent than riverine biomarkers, which are up to 3500 years older. The aged aeolian biomarkers are likely to result from admixture of lipids blown out of agriculturally degraded soils of the northern Black Sea catchment and lipids directly abraded from leaf surfaces. Preservation timescales of marine chloro- and pheopigments, which can directly be linked to photosynthesis, were estimated using radiocarbon ages of chlorophyll a, pheophytin a, pyropheophytin a, and cyclopheophorbide a enol as well as co-occurring TOC and bivalve shells from Black Sea core top sediments. Additionally, stable carbon and nitrogen isotopic compositions were determined to reconstruct the environmental conditions during the time of pigment synthesis. Since primary chloro- and pheopigments rapidly decompose in multiple ways including (autolytic) cell senescence, photo-oxidation, enzymatic or hydrolysis reactions, microbial and viral lysis, and grazing, their potential to be preserved as intact pigments in sediments is considered to be low. However, the radiocarbon concentrations of mainly phytoplanktonic chlorophyll a, pheophytin a, pyropheophytin a, and cyclopheophorbide a enol, which translate into ages of 40 up to 1200 years, imply preservation is much more efficient than expected. Preservation most likely results from mechanisms such as association with minerals or eutrophicationinduced hypoxia and light limitation. The stable nitrogen isotopic composition of the pigments indentifies nitrate utilization as the major nitrogenous nutrient uptake pathway especially at near-coast stations. Towards more offshore stations an isotopic depletion indicates N2-fixation as an additional nutrient utilization pathway. The long-term stable carbon and nitrogen isotopic variability appears to be as strong as the seasonal isotopic variations of the nutrient source, growth period, and habitat.



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Thesis (PhD)
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Eprint ID
37414
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Kusch, S. (2010): TRACING TIME IN THE OCEAN: UNRAVELING DEPOSITIONAL AND PRESERVATIONAL TIMESCALES USING COMPOUND-SPECIFIC RADIOCARBON ANALYSIS OF BIOMARKERS FROM MARINE SEDIMENTS PhD thesis, Universität Bremen.


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