Controls on the carbon isotopic composition of southern ocean phytoplankton

Abstract

Abstract. Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the •513C of Southern Ocean suspended particulate organic matter. Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role. It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (ep) on the reciprocal of CO2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on ep. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modem carbon cycle models that include phytoplankton carbon isotopic fractionation.

Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

“…In particular, the isotope composition of marine OC finds several applications in climate, ecology and carbon source apportionment studies. For example, stable carbon isotopes of marine phytoplankton are used for palaeo-pCO 2 reconstructions over geological timescales (Hoins et al, 2015;Pagani et al, 1999;Popp et al, 1999;Rau, 1994). The δ 13 C signature also provides a solid tool for marine food web and ecosystem structure investigations (Dunton et al, 2006;Iken et al, 2005;Kohlbach et al, 2016).…”

Novel analytical strategies for tracing the organic carbon cycle in marine and riverine particles

“…The isotopic depletion across the three stations follows the latitudinal gradient observed in other Southern Ocean studies, agreeing well with the relationship between the CO 2 supply and kinetic fractionation associated with photosynthesis. In the Southern Ocean, as euphotic zone waters grow colder and more CO 2(aq) -replete with latitude, carbon fixation by phytoplankton discriminates more in favor of 12 C (Goericke et al, 1994;Popp et al, 1999). In the Great Calcite Belt samples, we see euphotic zone water temperatures decrease from ~15-20°C to ~6°C (Table 1) from GB1-117 to GB2-43 while surface δ 13 C POC values decrease from ~-20‰ to ~-25‰.…”

Stable isotopes from diatom silica

“…Diatoms, in line with other organisms, preferentially uptake 12 C over 13 C. As photosynthetic carbon demand for diatoms and other organisms increases, a progressive depletion in 12 C DIC occurs, lowering δ 13 C DIC whilst leading to higher values of δ 13 C diatom (Laws et al, 1995(Laws et al, , 1997Bidigare et al, 1999;Rosenthal et al, 2000). The extent to which diatoms preferentially use 12 C over 13 C, however, is a function of CO 2(aq) concentration with an increase in CO 2(aq) leading to a decrease in δ 13 C diatom and vice-versa (Freeman and Hayes, 1992;Laws et al, 1995Laws et al, , 1997Rau et al, 1997;Popp et al, 1999). In interpreting records of δ 13 C diatom , further consideration is required as to the isotopic composition of the δ 13 C DIC substrate used during photosynthesis which may be altered by changes in ocean circulation, upwelling/stratification, the influx of terrestrial/riverine material and the dissolution of diatoms and other organisms in the photic zone, releasing 12 C enriched carbon into the water column.…”