Quantifying export production in the Southern Ocean: Implications for the Ba_xs proxy

Abstract

The water column and sedimentary Baxs distribution around the Crozet Plateau is used to decipher the controls and timing of barite formation and to evaluate how export production signals are recorded in sediments underlying a region of natural Fe fertilization within the Fe limited Southern Ocean. Export production estimated from preserved, vertical sedimentary Baxs accumulation rates are compared with published export fluxes assessed from an integrated study of the biological carbon pump to determine the validity of Baxs as a quantitative proxy under different Fe supply conditions typical of the Southern Ocean. Detailed assessment of the geochemical partitioning of Ba in sediments and the lithogenic end‐member allows appropriate correction of the bulk Ba content and determination of the Baxs content of sediments and suspended particles. The upper water column distribution of Baxs is extremely heterogeneous spatially and temporally. Organic carbon/Baxs ratios in deep traps from the Fe fertilized region are similar to other oceanic settings allowing quantification of the inferred carbon export based on established algorithms. There appears to be some decoupling of POC and Ba export in the Fe limited region south of the Plateau. The export production across the Crozet Plateau inferred from the Baxs sedimentary proxy indicates that the Fe fertilized area to the north of the Plateau experiences enhanced export relative to equivalent Southern Ocean settings throughout the Holocene and that this influence may also have impacted the site to the south for significant periods. This interpretation is corroborated by alternative productivity proxies (opal accumulation, 231Paxs/230Thxs). Baxs can be used to quantify export production in complex settings such as naturally Fe‐fertilized (volcanoclastic) areas, providing appropriate lithogenic correction is undertaken, and sediment focusing is corrected for along with evaluation of barite preservation.

Deep‐Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age

“…Opal is composed mainly of the frustules of diatoms, phytoplankton that produce much of the carbon exported from the surface ocean (Boyd & Trull, ; Buesseler et al, ), while xsBa (barite, BaSO 4 ) microcrystals are produced primarily in the thermocline within decomposing organic aggregates (e.g., Ganeshram et al, ). Barite is perhaps the best calibrated and most reliable inorganic indicator for past changes in the flux of organic carbon to the deep sea in pelagic environments (Hernandez‐Sanchez et al, ).…”

Paleoproductivity and Stratification Across the Subarctic Pacific Over Glacial‐Interglacial Cycles

“…For example, organic carbon is sensitive to oxygen concentrations in bottom and pore waters (e.g., Arndt et al, 2013;Ganeshram et al, 1999;Hedges et al, 1999), but its preservation is additionally affected by benthic activity and bioturbation (Canfield, 1994;Hartnett et al, 1998), host sediment composition (Keil et al, 1994;Keil & Hedges, 1993), and sedimentation rates (Müller & Suess, 1979). Excess barium, primarily in the form of barite (Dymond et al, 1992;Hernandez-Sanchez et al, 2011), is sensitive to sedimentary redox conditions (Dymond et al, 1992;Eagle et al, 2003;Hernandez-Sanchez et al, 2011;McManus et al, 1994McManus et al, , 1998Paytan & Griffith, 2007;Torres et al, 1996;van Os et al, 1991), such that under substantial sulfate reduction, barite dissolves and releases barium back to the pore waters, reducing or even eliminating any productivity record in that sedimentary proxy (Dickens, 2001;Dymond et al, 1992;Schenau et al, 2001). Dissolution of opal is omnipresent on the seafloor, since seawater is always undersaturated in dissolved silica, and thermodynamically, opal will dissolve until the pore water concentrations of silica reach equilibrium (Archer et al, 1993;Ragueneau et al, 2000).…”

Using the natural spatial pattern of marine productivity in the Subarctic North Pacific to evaluate paleoproductivity proxies

“…Previous studies have reported good correlations between Ba bio (or barite) and the flux of either organic carbon or of biogenic opal [e.g., Dehairs et al, 1980;Bishop, 1988;Dymond et al, 1992;François et al, 1995;Dymond and Collier, 1996;Paytan and Griffith, 2007]. Some controversy exists about the relationship of barite and Ba bio [Eagle et al, 2003;Robin et al, 2003], but recent studies have confirmed that Ba bio consists mainly of barite [e.g., Hernandez-Sanchez et al, 2011]. Barite is formed in the oxic microenvironments of decaying organic-rich particulate aggregates within the upper thermocline [e.g., Dehairs et al, 1980;Bishop, 1988;Dymond et al, 1992;François et al, 1995;Dymond and Collier, 1996;Paytan and Griffith, 2007].…”