Initial test of the silicic acid leakage hypothesis using sedimentary biomarkers

Abstract

Several mechanisms have been proposed for large CO2 changes at glacial Terminations, including shifting the CaCO3:Corg rain ratio by changing surface water nutrient supply, altering the balance between diatom and coccolithophore production. Diatom Si:N is highest in Fe‐stressed high‐latitude waters. Southern Ocean Fe enrichment studies suggest diatom Si demands reduced under Fe‐replete (glacial) conditions, allowing increased silicic acid to leak northward in subducted intermediate water and upwell at lower latitudes. We test this ‘Silicic Acid Leakage’ hypothesis using relative abundances of phytoplankton‐specific biomarkers in Peru margin sediments spanning 0–20 Ka. Results indicate increased coccolithophorid:diatom production from ∼0.5 to 3 between 18.0–15.5 Ka. Temporal correlation with the initial pCO2 rise and early deglacial shift in mode water ventilation implicates a coincidental, possibly causative reorganization of Sub‐Antarctic Mode Water formation and reduced Fe abundance. However, coccolithophorid production subsequently declined, suggesting rain ratio changes were only partly responsible for the CO2 deglacial transition.

Constraints on oceanic N balance/imbalance from sedimentary <sup>15</sup>N records

“…Qualitatively, the literature δ 15 N records supports a global oceanic impact from changes in water column denitrification; increasing (as a fraction of N 2 fixation) upon deglaciation with subsequent decrease perhaps due to a "slow" feedback mechanism increasing N 2 fixation or climatological forcing that again leads to decreases in water column denitrification. Peru margin δ 15 N records support the latter (Higginson and Altabet, 2004). If Fig.…”

Eastern Equatorial Pacific productivity and related-CO 2 changes since the last glacial period

“…Until now, the available paleoceanographic data have been inconclusive about changes in the CaCO 3 ∶organic carbon export ratio during glacial times in the EEP (14)(15)(16)(17)(18). A reason for the lack of unequivocal data is that these studies (except ref.…”

“…A reason for the lack of unequivocal data is that these studies (except ref. 16) focused only on the magnitude of diatom production instead of in its relative contribution to total export production compared with coccolithophores.…”

Increase in water column denitrification during the last deglaciation: the influence of oxygen demand in the eastern equatorial Pacific

“…whereas the records off Peru and Southern Mexico (Gulf of Tehuantepec) indicate a decrease during this time interval (Higginson and Altabet, 2004;Hendy and Pedersen, 2006). Such a Holocene δ 15 N decrease toward glacial values is not specific of the southern Mexican margin but has already been observed in other records within, proximal or even distal to denitrifying areas (Higginson and Altabet, 2004;Pride et al, 1999;Martinez et al, 2006;Hendy and Pedersen, 2006;Kao et al, Kienast et al, 2008;Pichevin et al, 2009).…”

“…A careful examination of the timing of δ 15 N maximum reached off Southern Mexico (Hendy and Pedersen, 2006), Costa Rica (this study), Peru (Higginson and Altabet, 2004) and Chile (De Pol-Holz et al, 2006Robinson et al, 2007;Martinez et al, 2006) is in fact difficult to assess and gives slightly different values: 15-15.5 ka off Costa Rica and Southern Mexico, 15.7-16.2 ka off Peru, 14.3-16.5 off Chile, whereas all this profiles show clearly a similar Antarctic timing. The age-model of all the cores is based on a combination of radiocarbon and oxygen isotope on benthic foraminifera, except off Peru where it is based on alkenones 14 C measurements because of the paucity or the absence of carbonate tests (Higginson and Altabet, 2004), and could be the main cause explaining these subtle differences, together with possible local processes. We can then assume that the denitrification profiles are all synchronized with a maximum occurring ∼15-16.5 ka.…”