Evidence for a major change in silicon cycling in the subarctic North Pacific at 2.73 Ma

Abstract

[1] The initiation of Northern Hemisphere glaciation in the subarctic North Pacific at $2.73 Ma was marked by an abrupt cessation of high opaline accumulation, considered to result from an increased stratification of the water column that should have led to higher utilization of nutrients in the surface ocean. We present a new stable Si isotope-based record of Si utilization that is hard to reconcile with this model. A drop in 30 Si/ 28 Si by 0.4% at 2.73 Ma is coincident with an increase in bulk N isotope composition. The contrasting utilization records cannot have been both caused by a hydrographic change alone. Excluding a change in the Si:N export ratio, these results either imply a relative increase in silicic acid supplied to the surface waters or a change in its Si isotope composition. While it is impossible to distinguish between these two possibilities, both imply a regional or global change in the Si biogeochemical cycle, potentially caused by an enhanced storage of Si in the underlying deep waters of the Pacific.

GEOTRACES inter-calibration of the stable silicon isotope composition of dissolved silicic acid in seawater

“…Besides their importance in understanding the present day Si cycle, δ 30 Si measurements are increasingly being used to assess past changes through the isotopic analysis of Si in biogenic silica within diatom frustules and within sponge spicules from marine sediments (e.g. Reynolds et al, 2008;Maier et al, 2013;Ehlert et al, 2015).…”

Contribution of changes in opal productivity and nutrient distribution in the coastal upwelling systems to Late Pliocene/Early Pleistocene climate cooling

“…Moreover, supposing a similar reorganization of the nutrient cycles between the North Pacific and the Southern Ocean during the extension of the polar ice cover and the stratification of the polar oceans ∼ 3.0-2.7 Ma, the decline in biogenic opal producers in both regions may have been accompanied by a reduced silicate utilisation in the Southern Ocean and thereby have led to lower δ 30 Si values comparable to those reconstructed in the North Pacific (from ∼ 1.6 to ∼ 1.2 ‰ around ∼ 2.73 Ma) (Reynolds et al, 2008). This implies that the isotopic signature of waters entering the BUS was most likely low or at least much lower than the values calculated for the surface waters in the BUS for the subsequent period between 3.0 and 2.0 Ma.…”

“…An anticorrelation between the δ 15 N and δ 30 Si records has also been described in the North Pacific (Reynolds et al, 2008) during the onset of significant Northern Hemisphere glaciation and the associated establishment of stratification of the surface waters at 2.73 Ma. A similar pattern has been found in the Southern Ocean during the last glacialinterglacial cycles (De La Rocha et al, 1998;Brzezinski et al, 2002).…”

“…Comparatively, in the North Pacific, the δ 15 N increased with the stratification of the surface waters at 2.73 Ma, while the δ 30 Si and BSi MARs declined. Reynolds et al (2008) argued that the inverse relationship between the proxies could not have been caused by an increase in Fe fertilisation as the opal productivity decreased instead of increased. They rather ascribed the anticorrelation to the combined sea ice and surface water stratification effects that limited the light and largely inhibited the upwelling of nutrients, thus causing a more complete N utilisation in this region compared with that of Si.…”

“…Sample aliquots were used to determine Si concentrations using a colorimetric method on a photospectrometer (Hansen and Koroleff, 1999). The chromatographic separation and purification (De Souza et al, 2012;Georg et al, 2006;Reynolds et al, 2008) was achieved with a precleaned 1 ml BioRad ion exchange column filled with AG50W-X8 (200-400 mesh) cation exchange resin. All δ 30 Si ratios were measured on a Nu instruments MC-ICPMS at GEOMAR in Kiel, which is equipped with an adjustable entrance slit for medium resolution to ensure peak separation of the 30 Si peak and the molecular interference of 14 N 16 O.…”