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Positive 14C gradients have recently been observed within the surface mixed layer of several continental‐margin sediments. The best explanation for these positive 14C gradients is the occurrence and rapid degradation of labile organic carbon (LOC) in the upper 5–10 cm of the seabed. Based on a two‐component model for sedimentary organic matter (i.e., a planktonic labile component and an older refractory component), bulk 14Corg data were used to determine the abundances of LOC within the surface mixed layers of three cores from the West Antarctic Peninsula (WAP) shelf and one core from San Clemente Basin (California Borderland). LOC contents in surface samples from the four stations varied from 0.5 to 1.1 mg/cm3, comprising 20% (San Clemente Basin) to 80% (WAP, Sta. G) of the total organic carbon. By incorporating a steady state diagenetic model and particle‐mixing bioturbation coefficients, the LOC profiles were used to determine LOC turnover times (LOC τ) and LOC e‐folding depths. The LOC τ values for the West Antarctic Peninsula sediments varied from 0.09 to 0.59 years, whereas the LOC τ value from the San Clemente Basin core was 63 years. The LOC e‐folding depths for the WAP stations varied from 0.8 to 3.4 cm, in contrast to the LOC e‐folding depth in San Clemente Basin, which was 4.0 cm. LOC characteristics from the four cores examined in this study were compared to LOC data in the literature as a means of substantiating the overall 14Corg‐based approach and justifying model assumptions. Plain Language Summary: A new C‐14 based technique for measuring the abundance of Labile (or reactive) Organic Carbon in marine sediments is presented. Based on this new approach and a model for sedimentary organic matter, Labile Organic Carbon depth profiles from West Antarctic Peninsula and California Borderland sediment cores were used to determine Labile Organic Carbon turnover times, which ranged from 0.1 to 0.6 years (Antarctic shelf sediments) to 63 years (California Borderland deposits). Within the bioturbated surface layer of the seabed, Labile Organic Carbon makes up only 6%–12% of the Total Organic Carbon, however, the Labile Organic Carbon is the source of nearly all nutrition and energy for bottom‐dwelling fauna and bacteria. The Labile Organic Carbon abundances and turnover times are in good agreement with values reported in the literature, confirming the efficacy of the new C‐14 based technique. Key Points: A new analytical technique, based on C‐14, is presented for assessing the distribution of labile organic carbon in marine sedimentsBased on samples from the Antarctic shelf and San Clemente Basin, seabed LOC distributions yielded τ values ranging from 0.1 to 60 yearsThe agreement between our LOC data and those reported in the literature provides confidence that the new technique is sound and justified [ABSTRACT FROM AUTHOR]

Abstract: Lipid, protein and carbohydrate concentrations have been determined in sediment cores from the continental shelf in the South Eastern Weddell Sea (SEWS), where no ice shelves have been present at least for thousands of years, and the continental shelf off the Eastern Antarctic Peninsula (EAP), in the area where two ice shelf collapses occurred in 1995 and 2002. On one hand, SEWS presents an important flux of fresh organic matter to the seabed during summer, whereas on the other hand, the presence of ice shelves in EAP hampered photosynthesis restricting the input of organic matter to advected refractory material. In the present study, biochemical variables and benthic macrofauna abundance, biomass and diversity confirmed differences between the two regions. Lipid concentrations were higher in SEWS than in EAP, whereas carbohydrate concentrations were higher in the latter region. These differences were attributed to the higher concentration of labile and refractory material, respectively. Biomass, abundance and diversity of the macrofauna were higher in SEWS than in EAP, where benthic communities started receiving a fresh organic matter input only after the recent ice shelf collapses. As regards macrofauna composition, both regions presented macrobenthic communities associated to early stages of recolonization. [Copyright &y& Elsevier]

Abstract: During austral summer 2006–2007, five sediment cores were recovered from the Eastern Antarctic Peninsula (EAP) continental shelf. Microscopic observations and sediment fatty acid (FA) composition analyses were carried out to investigate whether the drastic changes at the sea surface in EAP may be reflected in the sedimentary record. A sharp decrease in the number of diatom valves was observed below 2cm depth. This difference between the upper 2cm of sediment and the deeper part of the sediment column was attributed to the drastic change in the upper water column conditions after the collapse of the ice shelves, which allowed the arrival of phytoplankton debris and fresh organic matter to the sea floor in EAP. The presence of bacterial-, zooplankton- and detrital-related FA throughout EAP cores suggests that there has been an input of older and more refractory organic matter into the region, presumably by lateral transport before the Larsen ice shelves disintegration. [Copyright &y& Elsevier]

Abstract: Downward particle fluxes were collected on two sites on the Antarctic continental shelf (407 m to 491 m depth) during 30 days in the austral spring and summer 2003–2004. The sampling period included the onset and demise of a phytoplankton bloom and the occurrence of a storm. The calm period previous to the storm enabled the phytoplankton to flourish and set conditions for a bloom. The storm enhanced mixing, production and particle aggregation that increased the transport of organic matter to the seabed. The wind-induced particle pulse represented 53% (306 mg OC m−2 and 826 mg bSi m−2) of the total mass flux collected during the study period at midwater column (179 m depth). Close to the seabed (28 m above the seafloor) the sample corresponding to the period of the highest flux in the upper trap was lost; however, high OC and bSi fluxes (279 mg OC m−2 and 901 mg bSi m−2) were still collected three days after. These fluxes were 42% and 30% of the respective fluxes collected at this depth throughout the study. Diatom aggregates and faecal pellets were the main vehicles for the transport of organic matter. Faecal pellets and foraminifera tests were ubiquitous in the traps; however, ellipsoidal faecal pellets were only present in the deeper traps where in some periods were more abundant than the faecal strings. Comparatively smaller particle fluxes than in other Antarctic settings during the same season were found and the differences attributed to the denser sea ice coverage (>60%) observed during the present study. The chemical quality and the magnitude of the particle fluxes suggest that the phytoplankton bloom and the storm developed at least over a 75 km long area. The coincidence of these events produced in three days organic carbon and biogenic silica pulses over an area of at least 805 km2 of the continental shelf that may have amounted 85 to 225 tons and 308 to 725 tons, respectively. The occurrence of storms (>15 m s−1) during the spring–summer season in the last 20 years averaged 12 events per season, suggesting that the action of the wind could be very important in the development of organic matter-rich “green mats” on the sea floor. [Copyright &y& Elsevier]

Abstract: Within the SCAR''s international EASIZ programme, as part of the benthic–pelagic coupling experiment, grain size and organic matter contents in marine surface sediment were measured. Samples were taken during the austral autumn of 2000 from 3 regions in the eastern Weddell Sea: Kapp Norvegia, Four Seasons Bank, and Austasen. In general, sediments were fine sand with a grain size fraction <200 μm representing more than 40% of the total weight. The sediments from Four Seasons Bank (64 to 107 m depth) were coarser than those from Austasen and Kapp Norvegia (209 to 480 m depth), presumably due to winnowing of fine sediment at shallow depths. Organic carbon (OC) content ranged from 0.25% to 1.2% and constituted 10% to 97% of the total carbon. The samples from Kapp Norvegia presented the highest OC values. Overall, protein (PRT), lipid (LPD), and carbohydrate (CHO) contents were similar to those in sediment from cold regions (e.g., the North Atlantic and the Ross Sea) but higher than those in sediment from other Antarctic and more septentrional regions (e.g., the Ross Sea and the Mediterranean). The difference within the Antarctic is explained through the local conditions in Terra Nova Bay and Kapp Norvegia. In the Antarctic, PRT and LPD carbon were the main contributors to the biopolymeric carbon (BPC). In the eastern Weddell Sea shelf, the BPC accounted for more than 90% of the OC in most of the samples. More than 82% of the total PRT, LPD, and CHO were present in the fraction <200 μm. This work remarks the existence of sediments with a high nutritional value persistent several weeks after the spring–summer pulse of fresh organic matter. It is also highlighted the high potential availability of these sediments (due to its grain size) for the benthic communities inhabiting this high-latitude continental shelf. [Copyright &y& Elsevier]