Impacts of ocean acidification on sediment processes in shallow waters of the arctic ocean

Despite the important roles of shallow-water sediments in global biogeochemical cycling, the effects of ocean acidification on sedimentary processes have received relatively little attention. As high-latitude cold waters can absorb more CO2 and usually have a lower buffering capacity than warmer waters, acidification rates in these areas are faster than those in sub-tropical regions. The present study investigates the effects of ocean acidification on sediment composition, processes and sediment-water fluxes in an Arctic coastal system. Undisturbed sediment cores, exempt of large dwelling organisms, were collected, incubated for a period of 14 days, and subject to a gradient of pCO2 covering the range of values projected for the end of the century. On five occasions during the experimental period, the sediment cores were isolated for flux measurements (oxygen, alkalinity, dissolved inorganic carbon, ammonium, nitrate, nitrite, phosphate and silicate). At the end of the experimental period, denitrification rates were measured and sediment samples were taken at several depth intervals for solid-phase analyses. Most of the parameters and processes (i.e. mineralization, denitrification) investigated showed no relationship with the overlying seawater pH, suggesting that ocean acidification will have limited impacts on the microbial activity and associated sediment-water fluxes on Arctic shelves, in the absence of active bio-irrigating organisms. Only following a pH decrease of 1 pH unit, not foreseen in the coming 300 years, significant enhancements of calcium carbonate dissolution and anammox rates were observed. Longer-term experiments on different sediment types are still required to confirm the limited impact of ocean acidification on shallow Arctic sediment processes as observed in this study.

Shallow water marine sediment bacterial community shifts along a natural CO2 gradient in the Mediterranean Sea Off vulcano, Italy

The effects of increasing atmospheric CO(2) on ocean ecosystems are a major environmental concern, as rapid shoaling of the carbonate saturation horizon is exposing vast areas of marine sediments to corrosive waters worldwide. Natural CO(2) gradients off Vulcano, Italy, have revealed profound ecosystem changes along rocky shore habitats as carbonate saturation levels decrease, but no investigations have yet been made of the sedimentary habitat. Here, we sampled the upper 2 cm of volcanic sand in three zones, ambient (median pCO(2) 419 µatm, minimum Omega (arag) 3.77), moderately CO(2)-enriched (median pCO(2) 592 µatm, minimum Omega (arag) 2.96), and highly CO(2)-enriched (median pCO(2) 1611 µatm, minimum Omega (arag) 0.35). We tested the hypothesis that increasing levels of seawater pCO(2) would cause significant shifts in sediment bacterial community composition, as shown recently in epilithic biofilms at the study site. In this study, 454 pyrosequencing of the V1 to V3 region of the 16S rRNA gene revealed a shift in community composition with increasing pCO(2). The relative abundances of most of the dominant genera were unaffected by the pCO(2) gradient, although there were significant differences for some 5 % of the genera present (viz. Georgenia, Lutibacter, Photobacterium, Acinetobacter, and Paenibacillus), and Shannon Diversity was greatest in sediments subject to long-term acidification (>100 years). Overall, this supports the view that globally increased ocean pCO(2) will be associated with changes in sediment bacterial community composition but that most of these organisms are resilient. However, further work is required to assess whether these results apply to other types of coastal sediments and whether the changes in relative abundance of bacterial taxa that we observed can significantly alter the biogeochemical functions of marine sediments.

Planktonic foraminiferal assemblages of the Paleocene-Eocene Thermal Maximum of ODP Hole 113-690B

High-resolution study of Antarctic planktonic foraminiferal assemblages (Ocean Drilling Program Site 690, Weddell Sea) shows that these microplankton underwent a stepwise series of changes during the Paleocene-Eocene thermal maximum (PETM). Initiation of this response coincides with the onset of the carbon isotope excursion (CIE) but precedes the benthic foraminiferal mass extinction. The "top-to-bottom" succession in the biotic response indicates that the surface ocean/atmosphere was affected before the deep sea. The earliest stage of the faunal response entailed a conspicuous turnover within the shallow-dwelling genus Acarinina and a succession of stratigraphic first appearances. The genus Morozovella, large (>180 µm) biserial planktonics, and A. wilcoxensis are all restricted to the lower CIE within this PETM section. Acarininid populations crashed as the ocean/climate system ameliorated during the CIE recovery, reflecting atypical surface water conditions. This transient decline in acarininids is paralleled by a marked increase in carbonate content of sediments. It is postulated that this interval of carbonate enrichment, and its unusual microfauna, reflects enhanced carbon storage within reservoirs of the global carbon cycle other than the marine carbonate system (sensu Broecker et al., 1993, doi:10.1029/93PA00423; Ravizza et al., 2001, doi:10.1029/2000PA000541).

Pliocene-Pleistocene carbon isotope record of foraminifera from DSDP Site 89-586

Oceanographic changes in the western equatorial Pacific during the past 6 m.y. are inferred from carbon isotopic analyses of planktonic and benthic foraminifers from Ontong Java Plateau (DSDP Site 586). Sample spacing is 1.5 m (ca. 35,000-75,000 yr). An overall trend of d13C toward lighter values is evident for the last 5 m.y. in all four foraminiferal taxa analyzed (G. sacculifer, Pulleniatina, P. wuellerstorfi, and O. umbonatus). This trend is interpreted as an enrichment of the global ocean with 12C, because of the addition of carbon from organic carbon reservoirs (or lack of removal of carbon to such reservoirs), as a consequence of an overall drop in sea level. Differences between shallow- and deep-water d13C decrease slightly during this time interval, suggesting a moderate drop in productivity. This drop is not sufficient to explain the drop in sedimentation rate, however, much of which apparently must be ascribed to winnowing effects. A marked convergence in the d13C values of planktonic taxa exists within the last 2 m.y. We propose that this convergence indicates nutrient depletion in thermocline waters, caused by the vigorous removal of phosphate in marginal upwelling regions, or by the stripping of intermediate waters in their source regions. No large shifts are seen in the carbon isotope record of the last 6 m.y., in contrast to the oxygen isotope record. Some indication of cyclicity is present, with a period between 0.5 and 1.0 m.y. (especially in the earlier portion of the record).

Stable carbon and oxygen isotope ratios of benthic foraminifera

Carbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean delta13C values were 0.46 per mil lower during the last glacial maximum than during the Late Holocene. The geographic distribution of delta13C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high delta13C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean delta13C values during the glaciation are lower than both North Atlantic and Pacific delta13C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched 13C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean.

Geochemical analyses and Delta47 values in shallow water corals

Geochemical variations in shallow water corals provide a valuable archive of paleoclimatic information. However, biological effects can complicate the interpretation of these proxies, forcing their application to rely on empirical calibrations. Carbonate clumped isotope thermometry (Delta47) is a novel paleotemperature proxy based on the temperature dependent "clumping" of 13C-18O bonds. Similar ?47-temperature relationships in inorganically precipitated calcite and a suite of biogenic carbonates provide evidence that carbonate clumped isotope variability may record absolute temperature without a biological influence. However, large departures from expected values in the winter growth of a hermatypic coral provided early evidence for possible Delta47 vital effects. Here, we present the first systematic survey of Delta47 in shallow water corals. Sub-annual Red Sea Delta47 in two Porites corals shows a temperature dependence similar to inorganic precipitation experiments, but with a systematic offset toward higher Delta47 values that consistently underestimate temperature by ~8 °C. Additional analyses of Porites, Siderastrea, Astrangia and Caryophyllia corals argue against a number of potential mechanisms as the leading cause for this apparent Delta47 vital effect including: salinity, organic matter contamination, alteration during sampling, the presence or absence of symbionts, and interlaboratory differences in analytical protocols. However, intra- and inter-coral comparisons suggest that the deviation from expected Delta47 increases with calcification rate. Theoretical calculations suggest this apparent link with calcification rate is inconsistent with pH-dependent changes in dissolved inorganic carbon speciation and with kinetic effects associated with CO2 diffusion into the calcifying space. However, the link with calcification rate may be related to fractionation during the hydration/hydroxylation of CO2 within the calcifying space. Although the vital effects we describe will complicate the interpretation of Delta47 as a paleothermometer in shallow water corals, it may still be a valuable paleoclimate proxy, particularly when applied as part of a multi-proxy approach.

Seawater carbonate chemistry, virioplankton and bacterioplankton in a shallow CO2-dominated hydrothermal vent (Panarea Island, Tyrrhenian Sea)

Gas hydrothermal vents are used as a natural analogue for studying the effects of CO2 leakage from hypothetical shallow marine storage sites on benthic and pelagic systems. This study investigated the interrelationships between planktonic prokaryotes and viruses in the Panarea Islands hydrothermal system (southern Tyrrhenian Sea, Italy), especially their abundance, distribution and diversity. No difference in prokaryotic abundance was shown between high-CO2 and control sites. The community structure displayed differences between fumarolic field and the control, and between surface and bottom waters, the latter likely due to the presence of different water masses. Bacterial assemblages were qualitatively dominated by chemo- and photoautotrophic organisms, able to utilise both CO2 and H2S for their metabolic requirements. From significantly lower virioplankton abundance in the proximity of the exhalative area together with particularly low Virus-to-Prokaryotes Ratio, we inferred a reduced impact on prokaryotic abundance and proliferation. Even if the fate of viruses in this particular condition remains still unknown, we consider that lower viral abundance could reflect in enhancing the energy flow to higher trophic levels, thus largely influencing the overall functioning of the system.