Comparison of carbon and opal export rates between summer and spring bloom periods in the region of the Antarctic Polar Front, SE Atlantic

Abstract

Abstract Although primary production in the Antarctic Circumpolar Current is not above the world average and carbon burial rates are low, 70% of the world's opal burial occurs in this zone and it has been suggested that blooms of large diatoms are responsible for this extraordinary situation. Here we compare export fluxes during bloom and steady-state situations near the Antarctic Polar Front in the SE Atlantic. In a previous expedition during the austral spring, we observed the development of a bloom that led to the sudden export of particles (Deep-Sea Res. II 44 (1997) 457). Here we report the results of a second expedition to the same area in summer (December–January), 3 years later. 234 Th was monitored in the surface water and in Rosette casts down to a water depth of 500 m as tracer of export production in an intensive sampling program within a box of 275×375 km 2 . The distribution of particulate and dissolved 234 Th was remarkably constant over time and location. Total (dissolved+particulate) 234 Th activities were depleted relative to its parent 238 U at the surface ( 234 Th/ 238 U activity ratio approximately 83%), reaching equilibrium at a depth of around 190 m. This constant depletion corresponds to a 234 Th export rate of 1115 dpm/m 2 /d, 35% of the value observed during the spring bloom. Below 80 m depth, 234 Th on suspended particles correlates well with both particulate organic carbon (POC) and particulate biogenic silica (BSi), with a POC/ 234 Th ratio of 10.2±0.8 μmol/dpm and a BSi/ 234 Th ratio of 2.9±0.2 μmol/dpm. Using these ratios, the export from the upper 190 m is calculated to be 11.3 mmol C/m 2 /d and 3.2 mmol Si/m 2 /d. The export rates from the surface mixed layer (at 100 m depth) are 8.8 mmol C/m 2 /d and 2.5 mmol Si/m 2 /d. Thus, whereas in spring we had observed a sudden increase from a negligible to a high export rate, the summer situation around the Polar Front was characterized by a lower but steady export rate. This summer flux contributes a similar fraction to the annual export budget as the previously observed spring bloom, which has higher export rates but a much shorter duration.