Euphotic zone variations in bulk and compound‐specific δ13C of suspended organic matter in the Subantarctic Ocean, south of Australia

Abstract

The carbon isotopic compositions of suspended organic matter δ13CPOC collected from Subantarctic Zone surface waters south of Australia in November 1995 decrease southward from −20 to −26‰ and display strong correlations with aqueous carbon dioxide concentration ([CO2]aq), consistent with previous studies. In contrast, vertical profiles through the euphotic zone (top ∼100 m) of δ13CPOC at six stations display decreases with depth of up to 2.4‰. These decreases in δ13CPOC cannot be fully explained by the small vertical variations in [CO2]aq or its 13C content. Carbon 13 analyses of several individual sterols revealed similar isotopic changes with depth, suggesting that they derive from a fundamental depth control on primary production, rather than from algal community variations or remineralization processes. Growth rate μ appears to be the most likely source of the depth variations. The relationship between μ/[CO2]aq and δ13CPOC derived from surface water samples can explain the vertical variations of δ13CPOC within the mixed layer, provided integrated mixed layer growth rates are used. Below the mixed layer, differences between the observed δ13CPOC and the growth rate model can be explained by recent shallowing in mixed layer depth and the subsequent effect on growth rates. These results suggest that δ13CPOC determinations can be used to provide some information on the recent history of mixed layer processes and that interpretation of sedimentary δ13CPOC records should include consideration of possible growth rate and mixed layer depth variations.