Evidence for anthropogenic ¹⁴C-enrichment in estuarine waters adjacent to the North Sea

[1] The isotopic composition (d13C and D14C) of high molecular weight dissolved organic carbon (HMW DOC) was studied in the Tyne and Tweed estuaries, NE England. Despite significant removal of terrigenous HMW DOC in the low salinity regions (S < 15), D14C remained modern with little variation around 115%. This lack of apparent age discrimination was attributed to either non-oxidative removal or the absence of a significant proportion of old refractory C in the HMW DOC pool. At S < 15, we observed seaward increases in d13C and D14C. With no documented local 14C inputs, we attributed non-bomb related 14C-enrichment at S < 15 to a possible ‘lingering effect’ of distal anthropogenic sources in near-coastal North Sea HMW DOC. Given the global distribution of potential sources, we propose that anthropogenic 14C should be considered in assigning ages of DOC pools in near-coastal waters and suggest its possible use as a tracer for DOC transformations

Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment

A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ<sup>13</sup>C<sub>POC</sub>) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ<sup>13</sup>C<sub>POC</sub> in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of <i>Proboscia inermis</i> is strongly correlated with a large ~10 ‰ negative isotopic shift in δ<sup>13</sup>C<sub>POC</sub> that cannot be explained by a concurrent change in concentration or isotopic signature of CO<sub>2</sub>. We hypothesise that the δ<sup>13</sup>C<sub>POC</sub> shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ<sup>13</sup>C<sub>POC</sub> in <i>P. inermis</i> may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ<sup>13</sup>C<sub>POC</sub> may employ CCMs. These short-lived yet pronounced negative δ<sup>13</sup>C<sub>POC</sub> excursions drive a 4 ‰ decrease in the seasonal average δ<sup>13</sup>C<sub>POC</sub> signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 ‰ difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean δ<sup>13</sup>C<sub>POC</sub> variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ<sup>13</sup>C<sub>POC</sub>. We also find significantly higher δ<sup>13</sup>C<sub>POC</sub> in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO<sub>3</sub><sup>−</sup> and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ<sup>13</sup>C<sub>POC</sub> in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ<sup>13</sup>C<sub>POC</sub> in the sedimentary record

Anthropogenic nitrogen pollution threats and challenges to the health of South Asian coral reefs

Nitrogen pollution is a widespread and growing problem in the coastal waters of South Asia yet the ecological impacts on the region’s coral ecosystems are currently poorly known and understood. South Asia hosts just under 7% of global coral reef coverage but has experienced significant and widespread coral loss in recent decades. The extent to which this coral ecosystem decline at the regional scale can be attributed to the multiple threats posed by nitrogen pollution has been largely overlooked in the literature. Here, we assess the evidence for nitrogen pollution impacts on corals in the central Indian Ocean waters of India, Sri Lanka and the Maldives. We find that there is currently limited evidence with which to clearly demonstrate widespread impacts on coral reefs from nitrogen pollution, including from its interactions with other stressors such as seawater warming. However, this does not prove there are no significant impacts, but rather it reflects the paucity of appropriate observations and related understanding of the range of potential impacts of nitrogen pollution at individual, species and ecosystem levels. This situation presents significant research, management and conservation challenges given the wide acceptance that such pollution is problematic. Following from this, we recommend more systematic collection and sharing of robust observations, modelling and experimentation to provide the baseline on which to base prescient pollution control action

Ecological influences on {delta}¹³C of particulate matter in seasonally ice-covered Ryder Bay, Antarctica

Carbon isotopes may be a useful paleoceanographic tool for reconstructing past pCO2 of surface water, but isotopic composition of particulate organic carbon has been shown in both field and laboratory studies to be affected by pCO2, growth rate, cell size, cell geometry, light availability, carbon metabolism and species composition. To date, field studies have not constrained the dominant factors controlling surface water {delta}13CpOC. We present high-resolution time series data from a study in Ryder Bay, Antarctica, which compares seasonal fluctuations in mixed-layer {delta}13CpOC with detailed ecological and morphological analysis of phytoplankton communities, community productivity, {delta}13CDIC, nutrient dynamics, and hydrographic parameters. Preliminary results indicate that physical processes such as ocean-atmosphere gas exchange and upwelling do not significantly affect {delta}13CpOC signatures. In contrast, speciation shifts of diatom assemblages show strong correlation with changes in {delta} 13CpOC signals, supporting recent suggestions that taxonomic data are necessary for confident interpretation of sedimentary {delta}13C records

Variations in tropical convection as an amplifier of global climate change at the millennial scale

The global expression of millennial-scale climatic change during the glacial period and the persistence of this signal in Holocene records point to atmospheric teleconnections as the mechanism propagating rapid climate variations. We suggest rearrangements in the tropical convection system globally affected the concentration and location of atmospheric water vapour and modulated terrestrial and marine emissions of CH4 and N2O, providing a tropical mechanism of amplifying and perpetuating millennial-scale climate changes. A multi-proxy reconstruction reflecting various aspects of the intensity of the Arabian Sea Summer Monsoon shows strong millennial-scale variability over the past 90 kyr in which low intensity is associated with a southern shift of the Intertropical Convergence Zone (ITCZ) and an eastward shift in the equatorial convergence zone. The monsoon reconstruction, which is based on new data from a Somali margin sediment core, is supported by previously reported tropical paleoclimatic records and suggests that global scale millennial climatic variability is in part driven by modulations in the tropical hydrological cycle and tropical emissions of the greenhouse gases CH4 and N2O

Variation in production, input and preservation of metastable calcium carbonate off Somalia during the last 90,000 years

To improve our understanding of the Late Pleistocene and Holocene carbonate system of the western Arabian Sea a high-resolution sedimentary record off Somalia has been analysed. The 15.26-m-long piston core 905 comprises a complete record of the last 90,000 years. We have measured concentrations of carbonate minerals, i.e., aragonite, calcite, Mg-calcite, and element ratios (Sr/Ca) together with pteropod counts and an estimation of the preservation state of pteropod shells to trace temporal changes in carbonate production and preservation. The Sr/Ca ratio shows strong similarities to the aragonite percentage and the

Impacts of atmospheric anthropogenic nitrogen on the open ocean

Increasing quantities of atmospheric anthropogenic fixed nitrogen entering the open ocean could account for up to about a third of the ocean's external (nonrecycled) nitrogen supply and up to 3% of the annual new marine biological production, 0.3 petagram of carbon per year. This input could account for the production of up to 1.6 teragrams of nitrous oxide (N2O) per year. Although 10% of the ocean's drawdown of atmospheric anthropogenic carbon dioxide may result from this atmospheric nitrogen fertilization, leading to a decrease in radiative forcing, up to about two-thirds of this amount may be offset by the increase in N2O emissions. The effects of increasing atmospheric nitrogen deposition are expected to continue to grow in the future