Summer and winter distribution of δ¹³C_DIC in surface waters of the South Indian Ocean [20°S-60°S]

Abstract

This paper describes for the first time the summer and winter distributions of sea surface δ13CDIC in the Southern Indian Ocean (20°S–60°S). For this we used δ13CDIC measurements from 10 cruises conducted between 1998 and 2005. For summer and winter, the highest δ13CDIC values (>2‰) are observed in sub‐Antarctic waters (40°S–50°S) and attributed mainly to biological activity, enhanced in the vicinity of Crozet and Kerguelen Archipelagoes. The lowest δ13CDIC values are found in subtropical waters (25°S–35°S), with a minimum (<1‰) in the Agulhas Current region and in the Mozambique channel. On the seasonal scale, δ13CDIC is higher during summer than during winter in all regions. The largest seasonal amplitude of variation (∼0.3‰), observed in the region 35°S–40°S, is attributed to biological activity during summer and to deep vertical mixing during winter. In subtropical oligotrophic waters, the mean seasonal amplitude of variation (∼0.15‰) is mainly explained by air–sea CO2 fluxes. On the interannual scale, we also identified a large negative anomaly of δ13CDIC in the subtropical waters during austral summer 2002, associated to an anomalous ocean CO2 sink due to cold conditions during this period.

The effect of salinity on the biogeochemistry of the coccolithophores with implications for coccolith-based isotopic proxies

“…Thus, G. oceanica appears to be close (±0.5 ‰) to "equilibrium" conditions, whilst the other strains shift towards positive vital effects (up to +3.3 ‰ for RCC1212 E. huxleyi). These observations are also in good agreement with published literature (Hermoso, 2015;Hermoso et al, 2014;Holtz et al, 2015;McClelland et al, 2017;Rickaby et al, 2010). There seems to be, in contrary to the oxygen isotope system, an influence of salinity on coccolith δ 13 C values, at least for three of the four strains being examined here.…”

“…The proportion of CO 2aq and HCO − 3 assimilated by the cells primarily depends on ambient [CO 2aq ] and the inducible expression of active mechanisms to uptake carbon in the form of CO 2aq (excretion of carbonic anhydrase in the periplasmic environment) and HCO − 3 (transmembrane transporters). Within the cells, the residence of the DIC pool between assimilation and biomineralisation of DIC in the coccolith vesicle, and the relative allocation of the carbon resource into photosynthesis relative to calcification have large implications on the isotopic composition of coccolith calcite, both for the oxygen and carbon systems (Bolton and Stoll, 2013;Hermoso, 2014;Hermoso et al, 2016a, b;Holtz et al, 2015;McClelland et al, 2017;Rickaby et al, 2010).…”

The seasonal cycle of δ&lt;sup&gt;3&lt;/sup&gt;C&lt;sub&gt;DIC&lt;/sub&gt; in the North Atlantic subpolar gyre

“…In subtropical oceanic regions, the seasonal δ 13 C amplitude of 0.2-0.3 ‰ at the BATS (Bermuda Atlantic Time series) station and of 0.1 ‰ at the ALOHA station (Pacific time series north of Hawaii) reflect the local balance between air-sea CO 2 exchange, biological activity and vertical mixing (Gruber et al, 1998. This is also observed in the Southern Indian Ocean, with a seasonal δ 13 C amplitude of 0.3 ‰ in the western part of the basin (Racapé et al, 2010), whereas it reaches 0.45 ‰ south of Tasmania (McNeil and Tilbrook, 2009). Studies have also highlighted a significant decrease in δ 13 C DIC , attributed to the anthropogenic CO 2 uptake in the ocean.…”

“…Over the 2010-2012 period during the SURATLANT cruises, δ 13 C DIC were measured via an acid CO 2 extraction method in a vacuum system developed by Kroopnick (1974), whereas DIC were determined at the same time as total alkalinity (Alk) by a potentiometric titration derived from the method developed by Edmond (1970) using a closed cell and a CRM calibration. Further details on the sampling methods and analytical techniques are provided in Racapé et al (2010Racapé et al ( , 2013) for δ 13 C DIC and in Corbière et al (2007) for DIC. Both parameters were analyzed at the LOCEAN-IPSL Laboratory (Paris, France).…”

The seasonal cycle of δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;DIC&lt;/sub&gt; in the North Atlantic Subpolar Gyre

“…In subtropical oceanic regions, the seasonal δ 13 C amplitude of 0.2-0.3 ‰ at the BATS (Bermuda Atlantic Time series) station and of 0.1 ‰ at the ALOHA station (Pacific time series north of Hawaii) reflect the local balance between air-sea CO 2 exchange, biological activity and vertical mixing (Gruber et al, 1998(Gruber et al, , 1999Gruber and Keeling, 1999;. This is also observed in the Southern Indian Ocean, with a seasonal δ 13 C amplitude of 0.3 ‰ in the western part of the basin (Racapé et al, 2010), whereas it reaches 0.45 ‰ south of Tasmania (McNeil and Tilbrook, 2009). Studies have also highlighted a significant decrease in δ 13 C DIC , attributed to the anthropogenic CO 2 uptake in the ocean.…”