Search

We use a suite of eight ocean biogeochemical/ecological general circulation models from the Marine Ecosystem Model Intercomparison Project and Coupled Model Intercomparison Project Phase 5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO2 uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement on a dominance of either the SAM or the warming signal south of 44°S. In the southernmost zone, i.e., south of 58°S, they concur on an increase of biological export production, while between 44 and 58°S the models lack consensus on the sign of change in export. Yet in both regions, the models show an enhanced CO2 uptake during spring and summer. This is due to a larger CO2(aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44°S, all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44°S to the total uptake of the Southern Ocean south of 30°S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO2. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO2, with the climate-driven changes of natural CO2 exchange offsetting that trend only to a limited degree (∼10%) and with negligible impact of climate effects on anthropogenic CO2 uptake when integrated over a full annual cycle south of 30°S. [ABSTRACT FROM AUTHOR]

Abstract: As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90dpmm−2 d−1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2mmolCm−2 d−1 (2.1–9.0mmolCm−2 d−1) using POC/234Th ratio of bulk (respectively >50μm) suspended particles at the export depth (100m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements. [Copyright &y& Elsevier]

Abstract: We studied the water column distribution of total 234Th in subantarctic (SAZ) and polar front (PFZ) zone systems south of Tasmania during mid-austral summer 2007. The objective was to assess whether the observed zonal differences in biomass in this sector translated into variability of the carbon export and sequestration potential, and to identify possible causes inducing this variability. This study is part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Surface deficits in 234Th activities were observed at every station. 234Th export fluxes calculated from the 234Th activity deficits assuming steady state conditions showed higher 234Th export fluxes in the western than eastern SAZ, both of which were higher than those in the PFZ. 234Th fluxes sampled by free-drifting IRS and PPS-3 sediment traps at 150 and 170m during short-term deployments (∼ 6 days) at the three process stations were significantly lower than those obtained by the 234Th-deficit method. Possible reasons for this discrepancy are discussed. Carbon export fluxes were calculated based on the total 234Th fluxes (234Th deficit method) and the C:Th ratio for the >54μm particle size fraction from the appropriate export depth. The >54μm C:Th ratio was significantly lower in the eastern SAZ than the western SAZ or PFZ, resulting in carbon export fluxes that were lowest in the eastern SAZ. Overall, export fluxes range from 3.6±1.5 to 13.2±3.1mmol Cm−2 d−1. Carbon export fluxes are compared with gross primary production, new production and mesopelagic remineralization fluxes obtained by others during the same cruise. Contrary to expectations, we found higher export production in the PFZ and the western SAZ where biomass and dissolved Fe were lower than in the eastern SAZ. Significant differences in community structure of both primary producers and consumers likely contributed to this difference between the three regions. This pattern of higher shallow export continued into the mesopelagic, with lower remineralization efficiency in PFZ and SAZ-West compared to SAZ-East. Within the SAZ, east and west sites thus differed in their efficiency of carbon sequestration into the deep (>600m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. If the SAZ-East region can be used as an analog for a future climate warming scenario, our results suggest that export production at high latitudes could decrease, despite increased primary production. [Copyright &y& Elsevier]