Glacial-to-deglacial reservoir and ventilation ages on the southwest Iberian continental margin

[EN]Detailed assessments of past changes in surface and deep ocean reservoir ages are required to obtain robust 14C-based chronologies of planktic foraminifera and provide insights into ocean circulation changes and the C cycle. Here, we use plateau tuning on foraminiferal 14C data from a sediment core retrieved from the ‘Shackleton Sites’, a benchmark region for paleoceanographic studies, to i) develop a high-resolution record of surface water reservoir ages, ii) estimate “raw” apparent ventilation ages at two bottom water depths (3150 and 2650 mwd), and iii) establish robust age control for the last 23 ka. Our results provide new insights into the rapid changes in surface and deep-ocean reservoir ages that occurred over the last glacial maximum and last deglaciation. Marine reservoir ages contrast with previous estimates, especially for the cold spell Heinrich Stadial 1, and primarily reflect short-term changes in local hydrography. Variations in ventilation age indicate the influence of 14C-depleted, southern-source deep waters and a marked deepening of the settling depth of the highly ventilated Mediterranean Outflow Water during some millennial-scale intervals, much farther than previously assumed

The influence of lateral transport on sedimentary alkenone paleoproxy signals

[EN]Alkenone signatures preserved in marine sedimentary records are considered one of the most robust paleothermometers available and are often used as a proxy for paleoproductivity. However, important gaps remain regarding the provenance and fate of alkenones, as well as their impact on derived environmental signals in marine sediments. Here, we analyze the abundance, distribution and radiocarbon (14C) age of alkenones in bulk sediments and corresponding grain-size fractions in surficial sediments from seven continental margin settings in the Pacific and Atlantic oceans to evaluate the impact of organo-mineral associations and hydrodynamic sorting on sedimentary alkenone signals. We find that alkenones preferentially reside within fine-grained mineral fractions of continental margin sediments, with the preponderance of alkenones residing within the fine-silt fraction (2–10 µm) and most strongly influencing alkenone-14C age and sea surface temperature (SST) signals from bulk sediments as a consequence of their proportional abundance and higher degree of organic matter protection relative to other fractions. Our results provide further evidence for the key role of selective association of alkenones with mineral surfaces and associated hydrodynamic mineral sorting processes on the reliability of alkenone signals encoded in marine sediments (14C age, content and distribution) and the fidelity of corresponding proxy records (productivity and sea SST) in the spatial and temporal domain

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

Soil erosion plays a crucial role in transferring sediment and carbon from land to sea, yet little is known about the rhythm and rates of soil erosion prior to the most recent few centuries. Here we reconstruct a Holocene erosional history from central India, as integrated by the Godavari River in a sediment core from the Bay of Bengal. We quantify terrigenous fluxes, fingerprint sources for the lithogenic fraction and assess the age of the exported terrigenous carbon. Taken together, our data show that the monsoon decline in the late Holocene significantly increased soil erosion and the age of exported organic carbon. This acceleration of natural erosion was later exacerbated by the Neolithic adoption and Iron Age extensification of agriculture on the Deccan Plateau. Despite a constantly elevated sea level since the middle Holocene, this erosion acceleration led to a rapid growth of the continental margin. We conclude that in monsoon conditions aridity boosts rather than suppresses sediment and carbon export, acting as a monsoon erosional pump modulated by land cover conditions

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

The abundance of organic carbon (OC) in vegetation and soils (similar to 2,600 PgC) compared to carbon in the atmosphere (similar to 830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using bomb radiocarbon (C-14) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC C-14 signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79-83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000-1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins

What on Earth have we been burning? Deciphering sedimentary records of pyrogenic carbon

Humans have interacted with fire for thousands of years, yet the utilization of fossil fuels marked the beginning of a new era. Ubiquitous in the environment, pyrogenic carbon (PyC) arises from incomplete combustion of biomass and fossil fuels, forming a continuum of condensed aromatic structures. Here we develop and evaluate 14C records for two complementary PyC molecular markers, benzene-polycarboxylic-acids (BPCAs) and polycyclic-aromatic-hydrocarbons (PAHs) preserved in aquatic sediments from a sub-urban and a remote catchment in the United States (U.S.) from mid-1700s to 1998. Results show that the majority of PyC stems from local sources and is transferred to aquatic sedimentary archives on sub-decadal to millennial time scales. Whereas a small portion stems from near-contemporaneous production and sedimentation, the majority of PyC (<90%) experiences delayed transmission due to ‘pre-aging’ on millennial timescales in catchment soils prior to its ultimate deposition. BPCAs (soot) and PAHs (precursors of soot) trace fossil fuel-derived PyC. Both markers parallel historical records of the consumption of fossil fuels in U.S., yet never account for more than 19% total PyC. This study demonstrates that isotopic characterization of multiple tracers is necessary to constrain histories and inventories of PyC, and that sequestration of PyC can markedly lag its production

Temporal constraints on lateral organic matter transport along a coastal mud belt

Constraints on timescales of lateral transport of organic carbon (OC) over continental shelves and associated influences on the distribution and abundance of sedimentary OC remain sparse. Preferential degradation of labile, young OC during lateral transport results in apparent “diagenetic aging“ of OC. Additionally, sediment translocation can also result in ”transport time-associated aging“ of associated organic matter (OM) as a function of the lateral transport time (LTT). Here, we use a coupled thermal decomposition and radiocarbon (14C) approach to constrain timescales of lateral transport and concomitant loss of OC associated with different grain size fractions of sediments collected from two locations ∼275 km apart along a dispersal pathway on the inner shelf of the East China Sea. The 14C age contrasts between corresponding thermal fractions are used to distinguish these two components of sedimentary OM “aging”. To minimize interferences from hydrodynamic sorting and diagenetic aging of OC accompanying lateral transport, we assess 14C age differences of decomposition products from the most thermally-refractory OC components associated with specific grain size fractions between locations. We show that LTTs vary among different grain size fractions, and examine relationships between LTTs and sedimentary OC loss in order to assess the decomposition of OC as a consequence of lateral transport. We suggest that the decomposition of OC associated with protracted lateral transport exerts a strong influence on OC burial efficiency, with broad implications for carbon cycling over continental shelves

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 11997, doi:10.1038/s41598-018-30091-8.The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.We acknowledge funding support from the Agouron Institute Postdoctoral Fellowship (K.L.F), the US National Science Foundation (Awards: OCE-1333387 and OCE-13333826), the Investment in Science Fund given primarily by WHOI Trustee and Corporation Members, and the Swiss National Science Foundation (Award: 200020_163162)

An Abrupt Aging of Dissolved Organic Carbon in Large Arctic Rivers

Permafrost thaw in Arctic watersheds threatens to mobilize hitherto sequestered carbon. We examine the radiocarbon activity (F14C) of dissolved organic carbon (DOC) in the northern Mackenzie River basin. From 2003‐2017, DOC‐F14C signatures (1.00 ± 0.04; n = 39) tracked atmospheric 14CO2, indicating export of “modern” carbon. This trend was interrupted in June 2018 by the widespread release of aged DOC (0.85 ± 0.16, n = 28) measured across three separate catchment areas. Increased nitrate concentrations in June 2018 lead us to attribute this pulse of 14C‐depleted DOC to mobilization of previously frozen soil organic matter. We propose export through lateral perennial thaw zones occurred at the base of the active layer weakened by preceding warm summer and winter seasons. Although we are not yet able to ascertain the broader significance of this “anomalous” mobilization event, it highlights the potential for rapid and large‐scale release of aged carbon from permafrost

Compound-Specific Radiocarbon Analysis by Elemental Analyzer–Accelerator Mass Spectrometry: Precision and Limitations

[EN]We examine instrumental and methodological capabilities for microscale (10−50μg of C) radiocarbon analysisof individual compounds in the context of paleoclimate and paleoceanography applications, for which relatively high-precisionmeasurements are required. An extensive suite of data for14C-free and modern reference materials processed using differentmethods and acquired using an elemental-analyzer−accelerator-mass-spectrometry (EA-AMS) instrumental setup at ETHZurich was compiled to assess the reproducibility of specific isolation procedures. In order to determine the precision, accuracy,and reproducibility of measurements on processed compounds, we explore the results of both reference materials and threeclasses of compounds (fatty acids, alkenones, and amino acids) extracted from sediment samples. We utilize a MATLAB codedeveloped to systematically evaluate constant-contamination-model parameters, which in turn can be applied to measurementsof unknown process samples. This approach is computationally reliable and can be used for any blank assessment of small-sizeradiocarbon samples. Our results show that a conservative lower estimate of the sample sizes required to produce relativelyhigh-precision14C data (i.e., with acceptable errors of 0.5, a precision of 2% can be achieved for alkenone and fatty acid samples containing≥15 and 10μg of C, respectivel

Detrital Neodymium and (Radio)Carbon as Complementary Sedimentary Bedfellows? The Western Arctic Ocean as a Testbed

Interactions between organic and detrital mineral phases strongly influence both the dispersal and accumulation of terrestrial organic carbon (OC) in continental margin sediments. Yet the complex interplay among biological, chemical, and physical processes limits our understanding of how organo-mineral interactions evolve during sediment transfer and burial. In particular, diverse OC sources and complex hydrodynamic processes hinder the assessment of how the partnership of organic matter and its mineral host evolves during supply and dispersal over continental margins. In this study, we integrate new and compiled sedimentological (grain size, surface area), organic (%OC, OC-δ13C, OC-F14C), and inorganic isotopic (εNd, 87Sr/86Sr) geochemical data for a broad suite of surface sediments spanning the Western Arctic Ocean from the Bering Sea to the Mackenzie River Delta that capture diverse sources and ages of both terrestrial and marine material deposited in contrasting shelf and slope settings. Spatial gradients in sediment properties were used to delineate regional sources and transport processes influencing the dispersion and persistence of OC-mineral particle associations during export and burial. We found strong relationships between physical parameters, aluminum content, and OC-14C suggesting that terrestrial OC remains tightly associated with its detrital mineral carrier during source-to-sink transport. Notably, carbon and neodymium isotopic data yield consistent information regarding organic matter provenance. Results obtained highlight the potential for coupled organic-inorganic tracer measurements to elucidate sediment sources and to constrain physical and geochemical processes during sediment mobilization and transport in the Western Arctic Ocean. Tandem measurements of carbon and Nd isotopes may provide a new way to identify large-scale biogeochemical and ecological changes in the sources, nature, and fate of OC stemming from predicted increases in sea ice loss and fluvial inputs of dissolved and particulate OC to this complex and dynamic high latitude marginal sea