Testing the silicic acid leakage hypothesis: Records of opal burial from the equatorial Atlantic, equatorial Pacific and Southern Oceans over the past 30ka

• Main Author: Bradtmiller, Louisa I
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2008
• Subjects: Biogeochemistry; Geochemistry
• ISBN: 9780549854920; 0549854924

The Silicic Acid Leakage Hypothesis (SALH) suggests that during the Last Glacial Maximum (LGM) unused silicic acid escaped the Southern Ocean through intermediate and mode waters, and was transported to the equatorial oceans. Previous work indicates that in the presence of sufficient silicic acid, diatoms are better able to utilize nitrate and other nutrients than cocolithophorids, giving diatoms a competitive advantage. The potential implications of the SALH therefore include an ecological shift in tropical regions from cocolithophorids to diatoms during the LGM and, more importantly, a drawdown of CO2 from the atmosphere due to calcium carbonate compensation. The SALH also makes some testable predictions, namely that equatorial diatom productivity was increased during the LGM (relative to the Holocene), while Southern Ocean diatom productivity was reduced. This should be reflected in the sedimentary record as a change in biogenic opal burial, which has been shown in previous studies to be reliable proxy for diatom productivity. Downcore records of opal burial were measured in the equatorial Atlantic and Pacific Oceans, and in the Pacific sector of the Southern Ocean to test the SALH. In addition, 231Pa/230Th ratios measured on the same samples were used to discriminate between changes in productivity and changes in opal preservation. The equatorial Pacific data do not support the SALH, as there is only evidence for greater opal burial during the LGM in the two westernmost cores, while the remaining eight cores show lower LGM opal burial, confirmed by 231Pa/230Th ratios. The data support results from previous studies that invoke increased El Niño-like conditions during the LGM. Increased western Pacific opal flux and decreased opal flux in the EEP cold tongue during the LGM are both consistent with this idea. The equatorial Atlantic data are consistent with the SALH as all cores show increased LGM opal burial. Downcore records of 231Pa/230Th ratios are highly consistent with downcore opal records, implying that changes in opal burial reflect productivity and not dissolution. Cores from both basins show deglacial peaks in opal burial. Box estimates were used to compare the magnitude of the LGM-Holocene change in opal burial in the equatorial Atlantic and Pacific Oceans. In the equatorial Atlantic Ocean, glacial opal fluxes exceeded Holocene fluxes by approximately 1.8Gt opal/ka. Applying the same sized box to the eastern equatorial Pacific (the region of highest productivity) shows that Holocene fluxes there exceeded glacial fluxes by 1.8Gt opal/ka. While these are only first-order constraints on the silica budget, these results illustrate that the net glacial decrease in equatorial Pacific opal burial is of roughly equal magnitude to the increase in Atlantic opal burial during the same time. Total opal burial in the Southern Ocean was decreased (29%) during the LGM as compared to the Holocene. The spatial pattern of opal burial also differed between the two time periods; Holocene opal burial is greatest south of the Antarctic Polar Front (APF), while LGM opal burial was shifted north of the APF. However, the increase in glacial opal burial north of the APF was insufficient to offset decreased fluxes to the south, resulting in the observed net deficit in glacial opal burial in the Pacific SO. When averages of our fluxes (weighted by the area represented) were applied over the 60°W-140°E (the range of opal accumulation found in our samples), this yielded Holocene and LGM burial rates of 53.0 and 37.8 Gt opal/kyr, respectively. The difference, 15.2 Gt opal/kyr, is larger than the burial of opal in the equatorial Atlantic and Pacific oceans combined, and is an order of magnitude larger than the observed glacial-interglacial changes in equatorial opal burial. Our data show a 29% decrease in SO opal burial during the last glacial period, which satisfies the primary requirement of the SALH. However, evidence from the equatorial oceans does not support the subsequent predictions of the SALH, namely that either opal burial or the Corg:CaCO 3 ratio should have increased in the glacial tropics. The magnitude of changes in equatorial opal burial are an order of magnitude smaller than the projected total export of Si from the SO, and are offsetting between the Atlantic and Pacific basins. This suggests that other factors, perhaps changes in upwelling or intermediate water circulation, are likely responsible for observed changes in equatorial opal burial during the LGP. Furthermore, our data suggest that there exists an unidentified glacial sink for Si, possibly along the continental margins. The existence of this sink, and its possible effects on glacial CO2, remain an interesting topic for future investigation. While the absence of evidence in support of the SALH does not constitute ‘evidence of absence’, the sum of paleo evidence to date from the tropical and Southern oceans does not support the predictions of this hypothesis.