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

¹⁴C AMS at SUERC: improving QA data from the 5 MV tandem AMS and 250 kV SSAMS

In 2003, a National Electrostatics Corporation (NEC) 5MV tandem accelerator mass spectrometer was installed at SUERC, providing the radiocarbon laboratory with 14C measurements to 4–5‰ repeatability. In 2007, a 250kV single-stage accelerator mass spectrometer (SSAMS) was added to provide additional 14C capability and is now the preferred system for 14C analysis. Changes to the technology and to our operations are evident in our copious quality assurance data: typically, we now use the 134-position MC-SNICS source, which is filled to capacity. Measurement of standards shows that spectrometer running without the complication of on-line δ13C evaluation is a good operational compromise. Currently, 3‰ 14C/13C measurements are routinely achieved for samples up to nearly 3 half-lives old by consistent sample preparation and an automated data acquisition algorithm with sample random access for measurement repeats. Background and known-age standard data are presented for the period 2003–2008 for the 5MV system and 2007–2008 for the SSAMS, to demonstrate the improvements in data quality

Nutrient cycling in the Atlantic basin: the evolution of nitrate isotope signatures in water masses

A basin-wide transect of nitrate isotopes (δ15NNO3, δ18ONO3), across the UK GEOTRACES 40°S transect in the South Atlantic is presented. This data set is used to investigate Atlantic nutrient cycling and the communication pathways of nitrogen cycling processes in the global ocean. Intermediate waters formed in the subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilization of nitrate by phytoplankton and distant denitrification processes, transporting heavy isotope signatures to the subtropical Atlantic. Water mass modification through the Atlantic is investigated by comparing data from 40°S (South Atlantic) and 30°N (North Atlantic). This reveals that nitrate in the upper intermediate waters is regenerated as it transits through the subtropical Atlantic, as evidenced by decreases in δ18ONO3. We document diazotrophy-producing high N:P particle ratios (18–21:1) for remineralization, which is further confirmed by a decrease in δ15NNO3 through the subtropical Atlantic. Thesemodifications influence the isotopic signatures of the North Atlantic Deep Water (NADW) which is subsequently exported from the Atlantic to the Southern Ocean. This study reveals the dominance of recycling processes and diazotrophy on nitrate cycling in the Atlantic. These processes provide a source of low δ15NNO3 to the Southern Ocean via the NADW, to counteract enrichment in δ15NNO3 from water column denitrification in the Indo/Pacific basins. We hence identify the Southern Ocean as a key hub through which denitrification and N2 fixation communicate in the ocean through deepwater masses. Therefore, the balancing of the oceanic N budget and isotopic signatures require time scales of oceanic mixing

Positive ion AMS with single stage accelerator and RF-plasma ion source at SUERC

The new single-stageaccelerator mass spectrometer at SUERC can also accept positiveions. Potential benefits of positiveion measurement with suitable ionsources are more convenient sample form, smaller sample size, a variety of available charge states and maybe new applications. We have begun with oxygen isotope analyses to demonstrate destruction of interfering molecules

Single-stage accelerator mass spectrometer radiocarbon-interference identification and positive-ionisation characterisation

A single-stageaccelerator mass spectrometer (SSAMS) is a good alternative to conventional spectrometers based on tandem electrostatic acceleration for radiocarbon measurement and permits experimentation with both negative and positive carbon ions. However, such <sup>14</sup>C AMS of either polarity ions is limited by an interference. In the case of anion acceleration we have newly determined this to be summed <sup>13</sup>C and <sup>16</sup>O by improvising an additional Wien filter on our SSAMS deck. Also, <sup>14</sup>C AMS might be improved by removing its dependency on negative-ionisation in a sputter ion source. This requires negative-ionisation of sample atoms elsewhere to suppress the <sup>14</sup>N interference, which we accomplish by transmitting initially positive ions through a thin membrane. The ionisation dependence on ion-energy is found to be consistent with previous experimentation with vapours and thicker foils

Attempted positive ion radiocarbon AMS

The SUERC single-stage accelerator mass spectrometer is capable of generating and accelerating-fromground positive ions. Having previously shown suppression of molecular interferences we have proceeded to study positive ion radiocarbon AMS. Potential benefits are convenient and efficient ion production. This depends on overcoming the large <sup>14</sup>N atomic isobar which we have suppressed by seven orders of magnitude in the accelerator terminal. A remaining interference is more problematic, however

Hydrogen isotope analysis of natural abundance and deuterium- enriched waters by reduction over chromium on-line to a dynamic dual inlet isotope-ratio mass spectrometer

This paper describes the application of a simple chromium reduction furnace which can be interfaced with a dual inlet isotope-ratio mass spectrometer thus providing the capacity for cheap, fast, accurate and precise measurement of deltaD(V-SMOW) by dynamic mass spectrometry. Measurements are precise to the order of less than or equal to0.5%. Mean 95% confidence intervals for the Vienna Standard Mean Ocean Water (V-SMOW) to Standard Light Antarctic Precipitation (SLAP) range are in the order of 2.5 parts per thousand and the system is linear over the range -428 to 23,000 parts per thousand. Memory effects do exist, but are small for natural abundance samples and can be minimised by careful planning of the analytical load

Performance of the new single stage accelerator mass spectrometer at the SUERC

A NEC 250 kV single stage accelerator mass spectrometer (SSAMS) has been installed along-side our five year old 5 MV spectrometer. The new instrument is intended to take over routine radiocarbon measurement, thereby providing more capacity for our rapidly growing cosmogenic nuclide programme. Initial tests show that although flawed the SSAMS sufficiently mimics the good performance of the first machine; high-precision 14C/13C measurement is being achieved with a background limited by sample preparation

Improved ³⁶Cl AMS at 5MV

Small changes to our ion source and gas ionization detector have significantly improved Cl measurement by reducing source memory and increasing interference rejection. Gas stripped low energy 30 MeV <sup>36</sup>Cl<sup>5+</sup> ions are still efficiently transported to the detector but <sup>36</sup>S vs. <sup>36</sup>Cl separation is improved by an order of magnitude. Accordingly <sup>36</sup>Cl/Cl background is <10<sup>-15</sup> before additional interference correction that is also newly automated. <sup>35</sup>Cl currents have increased to 30 μA and <sup>36</sup>Cl/Cl inter-cathode repeatability is 3%

Carbon carbon detection: diamond detectors and AMS

14C ions (290 keV) have been detected with chemically vapour deposited diamond. Potential benefits of diamond detectors are radiation hardness, high charge collection and very fast response time/high bandwidth