Storage and hydrolysis of seawater samples for inorganic carbon isotope analysis

Preservation of seawater samples was tested for total inorganic carbon (ΣCO2), stable carbon isotope (δ13C), and radiocarbon (14C) applications using foil bags and storage by refrigeration and freezing. The aim was to preserve representative samples with minimal storage effects but without using toxic methods such as mercuric chloride poisoning. Hydrolysis of samples to CO2 was based on existing methods. Results of IAEA-C2 standard used with deionized water stored in the foil bags showed complete reaction yields, 14C results within 2σ of the consensus value, and δ13C that were internally consistent, indicating that there were no procedural effects associated with the foil bags. 14C results were statistically indistinguishable across the storage times, for frozen and refrigerated seawater samples from a coastal site, Elie Ness, Fife, UK. The scatter of ΣCO2 concentrations and δ13C was within scatter observed in other studies for lake- and seawater samples preserved by acidification or using mercuric chloride. However, both ΣCO2 and δ13C were less variable for frozen samples compared with refrigerated samples. The foil bags are lighter, safer to transport, and similar in cost to glass bottles and allow sample collection in the field and transfer to the hydrolysis vessel without exposure of the sample to atmosphere. Storage of seawater samples in the foil bags was considered a reliable, alternative method to poisoning for ΣCO2, δ13C, and 14C, and freezing the samples is recommended for storage time beyond a week

Isotopic fractionation of carbon during uptake by phytoplankton across the South Atlantic subtropical convergence

The stable isotopic composition of particulate organic carbon (δ13CPOC) in the surface waters of the global ocean can vary with the aqueous CO2 concentration ([CO2(aq)]) and affects the trophic transfer of carbon isotopes in the marine food web. Other factors such as cell size, growth rate and carbon concentrating mechanisms decouple this observed correlation. Here, the variability in δ13CPOC is investigated in surface waters across the south subtropical convergence (SSTC) in the Atlantic Ocean, to determine carbon isotope fractionation (ϵp) by phytoplankton and the contrasting mechanisms of carbon uptake in the subantarctic and subtropical water masses. Our results indicate that cell size is the primary determinant of δ13CPOC across the Atlantic SSTC in summer. Combining cell size estimates with CO2 concentrations, we can accurately estimate "p within the varying surface water masses in this region. We further utilize these results to investigate future changes in "p with increased anthropogenic carbon availability. Our results suggest that smaller cells, which are prevalent in the subtropical ocean, will respond less to increased [CO2(aq)] than the larger cells found south of the SSTC and in the wider Southern Ocean. In the subantarctic water masses, isotopic fractionation during carbon uptake will likely increase, both with increasing CO2 availability to the cell, but also if increased stratification leads to decreases in average community cell size. Coupled with decreasing δ13C of [CO2(aq)] due to anthropogenic CO2 emissions, this change in isotopic fractionation and lowering of δ13CPOC may propagate through the marine food web, with implications for the use of δ13CPOC as a tracer of dietary sources in the marine environment

Tracing the role of Arctic shelf processes in Si and N cycling and export through the Fram Strait: insights from combined silicon and nitrate isotopes

Nutrient cycles in the Arctic Ocean are being altered by changing hydrography, increasing riverine inputs, glacial melt and sea-ice loss due to climate change. In this study, combined isotopic measurements of dissolved nitrate (δ15N-NO3 and δ18O-NO3) and silicic acid (δ30Si(OH)4) are used to understand the pathways that major nutrients follow through the Arctic Ocean. Atlantic waters were found to be isotopically lighter (δ30Si(OH)4=+ 1.74 ‰) than their polar counterpart (δ30Si(OH)4=+ 1.85 ‰) owing to partial biological utilisation of dissolved Si (DSi) within the Arctic Ocean. Coupled partial benthic denitrification and nitrification on Eurasian Arctic shelves lead to the enrichment of δ15N-NO3 and lighter δ18O-NO3 in the polar surface waters (δ15N-NO3= 5.44 ‰, δ18O-NO3= 1.22 ‰) relative to Atlantic waters (δ15N-NO3= 5.18 ‰, δ18O-NO3= 2.33 ‰). Using a pan-Arctic DSi isotope dataset, we find that the input of isotopically light δ30Si(OH)4 by Arctic rivers and the subsequent partial biological uptake and biogenic Si burial on Eurasian shelves are the key processes that generate the enriched isotopic signatures of DSi exported through Fram Strait. A similar analysis of δ15N-NO3 highlights the role of N-limitation due to denitrification losses on Arctic shelves in generating the excess dissolved silicon exported through Fram Strait. We estimate that around 40 % of DSi exported in polar surface waters through Fram Strait is of riverine origin. As the Arctic Ocean is broadly N-limited and riverine sources of DSi are increasing faster than nitrogen inputs, a larger silicic acid export through the Fram Strait is expected in the future. Arctic riverine inputs therefore have the potential to modify the North Atlantic DSi budget and are expected to become more important than variable Pacific and glacial DSi sources over the coming decades.</p

Silica cycling and isotopic composition in northern Marguerite Bay on the rapidly-warming western Antarctic Peninsula

The Southern Ocean is a key region for silica (Si) cycling, and the isotopic signatures established here influence the rest of the world's oceans. The climate and ecosystem of the Southern Ocean are changing rapidly, with the potential to impact Si cycling and isotope dynamics. This study examines high-resolution time-series dataset of dissolved Si concentrations and isotopic signatures, particulate Si concentrations and diatom speciation at a coastal site on the western Antarctic Peninsula (WAP), in order to characterise changes in Si cycling with respect to changes occurring in productivity and diatom assemblages. Dissolved and particulate Si phases reflect the dominant control of biological uptake, and combined with isotopic fractionation were consistent with a season of low/intermediate productivity. Biogenic Si is tightly coupled to both chlorophyll and particulate organic carbon at the sampling site, consistent with diatom-dominated phytoplankton assemblages along the WAP. Variability in diatom speciation has a negligible impact on the isotopic signature of dissolved Si in surface waters, although this is unlikely to hold for sediments due to differential dissolution of diatom species. A continued decline in diatom productivity along the WAP would likely result in an increasing unused Si inventory, which can potentially feed back into Si-limited areas, promoting diatom growth and carbon drawdown further afield

Silicic acid biogeochemistry in the Gulf of California: Insights from sedimentary Si isotopes

Iron is considered to play a large role in the cycling of Si in Fe-limited regions of the ocean, but little is known about its role in Si biogeochemistry outside these areas. Here, we present published sediment trap data, new nutrient profiles and high resolution sedimentary records (Si isotopes, Biogenic silica%, N% and C%) from the Gulf of California, a non-Fe-limited region, to investigate the history of Si cycling in this highly productive basin. Modern nutrient profiles show that silicic acid in subsurface waters is in excess relative to nitrate and is therefore incompletely utilized during moderate winter upwelling events. Modern data, however, suggest that during intense upwelling episodes, silicic acid is preferentially utilized relative to nitrate by the biota, which we suggest reflects transient iron limitation. Our new delta Si-30 record from the Guaymas Basin shows dramatic variations at millennial timescales. Low delta Si-30 values synchronous with Heinrich events are interpreted as resulting from the decline in Si(OH)(4) utilization at times of decreased upwelling strength, while nearly complete Si(OH)(4) utilization was observed at times of invigorated upwelling and increased opal burial during the Holocene, the Bolling-Allerod and the last glacial period. We attribute the complete utilization of Si(OH)(4) to the occurrence of transient Fe limitation at these times. Our study highlights the importance of Fe limitation on Si and C cycling in coastal upwelling regions and suggests that upwelling dynamics, in combination with Fe availability, have the potential to modulate marine Si distribution and opal burial even at short timescales.KEYWORDS: North Pacific, Coastal upwelling regimes, Climate change, Marine diatoms, Biogenic silica, Guaymas basin, Equatorial Pacific, Iron, Late quaternary, Southern ocea

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

Interhemispheric leakage of isotopically heavy nitrate in the eastern tropical Pacific during the last glacial period

International audienceWe present new high-resolution N isotope records from the Gulf of Tehuantepec and the Nicaragua Basin spanning the last 50-70 ka. The Tehuantepec site is situated within the core of the north subtropical denitrification zone while the Nicaragua site is at the southern boundary. The delta N-15 record from Nicaragua shows an ``Antarctic'' timing similar to denitrification changes observed off Peru-Chile but is radically different from the northern records. We attribute this to the leakage of isotopically heavy nitrate from the South Pacific oxygen minimum zone (OMZ) into the Nicaragua Basin. The Nicaragua record leads the other eastern tropical North Pacific (ETNP) records by about 1000 years because denitrification peaks in the eastern tropical South Pacific (ETSP) before denitrification starts to increase in the Northern Hemisphere OMZ, i.e., during warming episodes in Antarctica. We find that the influence of the heavy nitrate leakage from the ETSP is still noticeable, although attenuated, in the Gulf of Tehuantepec record, particularly at the end of the Heinrich events, and tends to alter the recording of millennial timescale denitrification changes in the ETNP. This implies (1) that sedimentary delta N-15 records from the southern parts of the ETNP cannot be used straightforwardly as a proxy for local denitrification and (2) that denitrification history in the ETNP, like in the Arabian Sea, is synchronous with Greenland temperature changes. These observations reinforce the conclusion that on millennial timescales during the last ice age, denitrification in the ETNP is strongly influenced by climatic variations that originated in the highlatitude North Atlantic region, while commensurate changes in Southern Ocean hydrography more directly, and slightly earlier, affected oxygen concentrations in the ETSP. Furthermore, the delta N-15 records imply ongoing physical communication across the equator in the shallow subsurface continuously over the last 50-70 ka

Comparative roles of upwelling and glacial iron sources in Ryder Bay, coastal western Antarctic Peninsula

Iron (Fe) is an essential micronutrient for phytoplankton, and is scarce in many regions including the open Southern Ocean. The western Antarctic Peninsula (WAP), an important source region of Fe to the wider Southern Ocean, is also the fastest warming region of the southern hemisphere. The relative importance of glacial versus marine Fe sources is currently poorly constrained, hindering projections of how changing oceanic circulation, productivity, and glacial dynamics may affect the balance of Fe sources in this region.Dissolved and total dissolvable Fe concentrations were measured throughout the summer bloom period at a coastal site on the WAP. Iron inputs to the surface mixed layer in early summer were strongly correlated with meteoric meltwater from glaciers and precipitation. A significant source of Fe from underlying waters was also identified, with dissolved Fe concentrations of up to 9.5 nM at 200 m depth. These two primary Fe sources act on different timescales, with glacial sources supplying Fe during the warm summer growing period, and deep water replenishing Fe over annual periods via deep winter mixing.Iron supply from deep water is sufficient to meet biological demand relative to macronutrient supply, making Fe limitation unlikely in this area even without additional summer Fe inputs from glacial sources. Both glacial and deep-water Fe sources may increase with continued climate warming, potentially enhancing the role of the WAP as an Fe source to offshore waters