The Si cycle in the Pacific sector of the Southern Ocean: seasonal diatom production in the surface layer and export to the deep sea

Abstract

Abstract Dissolved silicic acid (Si(OH) 4 ) and biogenic silica (BSiO 2 ) concentrations were measured on four different cruises in the Southern Ocean along 170°W from late October 1997 to mid-March 1998. These data were used to construct a vertically integrated Si budget in the surface layer from 58° to 68°S. Throughout that period there was a distinct, southward-moving meridional [Si(OH) 4 ] gradient, with concentrations increasing to the south. In October the gradient was centered on the Antarctic Polar Front (APF) at 61°S. Surface layer [Si(OH) 4 ] increased from 10 to >40 mmol m −3 between 58° and 62°S, while BSiO 2 concentrations were −3 and −2 throughout the study area. As the surface mixed layer stratified and shoaled in December, diatoms bloomed in response to the more favorable light conditions, consuming Si(OH) 4 in the process. By late December BSiO 2 concentrations were exceptionally high for the open ocean, with maximum integrated values of 700 mmol Si m −2 around 62°S, while vertically integrated [Si(OH) 4 ] in the upper 50 m had dropped from 2500 to 870 mmol m −2 , an average decrease of >30 mmol Si m −3 . The diatom bloom persisted into January with maximum integrated BSiO 2 of 600 mmol m −2 between 64° and 65°S, while [Si(OH) 4 ] was depleted to −3 in surface waters as far south as 64.5°S. The center of the [Si(OH) 4 ] gradient had moved southward to 65°S by late January, coincident with maximal diatom biomass at that time. The bloom ended by March with the disappearance of most of the diatom biomass from the surface layer, deeper mixed layers, and increased [Si(OH) 4 ]. By late January, diatom production had removed >70% of the of Si(OH) 4 initially present in the upper 50 m everywhere north of 65°S, and had displaced the [Si(OH) 4 ] gradient >400 km southward, while the APF remained stationary. Between November and late January (∼80 days), an average of 2200 mmol Si m −2 had been removed from the upper 50 m between 61° and 65°S. Based on changes in vertically integrated Si(OH) 4 and BSiO 2 concentrations between November and March, an average of 2 mol Si m −2 was exported from the upper 50 m between 61° and 65°S, with 59% of that export occurring between late December and late January. The observed seasonal export is very high in comparison with other oceans, and the high export coincides spatially with a well-documented zone of high opal flux to 1000 m and high opal content in modern sediments. North of 61°S BSiO 2 export was limited by the low initial standing stock of Si(OH) 4 in surface waters, and south of 65°S export was likely limited by the shorter growing season. Thus, the combined effect of a large initial supply of Si(OH) 4 in surface waters and conditions that permit efficient use of that Si by diatoms appears to be the primary cause of opal-rich sediment accumulation in the Pacific sector of the Southern Ocean.