Sedimentary opal records in the eastern equatorial Pacific: It is not all about leakage

Abstract

[1] The clear predictions of the silicic acid leakage hypothesis (SALH) resulted in a number of studies of downcore opal records from the tropical Pacific. The original SALH predicts that unused silicic acid, due to Fe-driven changes in Si versus N limitation, escaped from the glacial Southern Ocean to equatorial upwelling regimes where it enhanced diatom productivity, thereby decreasing coccolith growth and lowering atmospheric CO 2 . In contrast to SALH predictions, however, sedimentary records from the eastern equatorial Pacific (EEP) do not show enhanced opal burial during the Last Glacial Maximum (LGM) but higher rates of opal burial during the deglaciation and marine isotopic stage 3 (MIS3). The peak in opal productivity during the deglaciation has been attributed to increased supply of nutrient-rich waters driven by stronger upwelling of deep water in the Southern Ocean at the end of last glacial period. The large peak in opal burial observed in a number of EEP cores during MIS3 was interpreted as evidence for Si leakage when Southern Ocean diatom productivity was limited by both low dust flux and extended sea ice. On the other hand, the paradoxical LGM decline in opal accumulation in the EEP was explained by enhanced dust input that lowered the diatom Si:C uptake ratio. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to revisit opal burial in the EEP, in particular during the MIS3 "opal peak." An increase in algal productivity is not supported by the sedimentary concentration of brassicasterol, an organic molecule commonly found in diatoms, or by the ratio of ( 231 Pa/ 230 Th) xs,0 , a proxy for opal export production. We therefore conclude that the large peak in opal burial during MIS3 reflects enhanced preservation of diatoms. Building on mechanisms invoked in previous studies, we hypothesize that opal burial in the EEP is controlled both by the physiological response of diatoms to low-latitude Fe inputs and by the high-latitude processes leading to silicic acid leakage.

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

“…An enhanced biological pump has also been invoked to operate in the EEP during the last glacial maximum based on a silicon isotope record (31). So far, most of the attempts to test the SALH were based on opal records from the tropical Pacific (14,15,17,18). These studies challenged the SALH on the basis of a decrease in opal accumulation during glacial times.…”

“…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.…”

Glacial deep ocean sequestration of CO2 driven by the eastern equatorial Pacific biologic pump

“…There remains considerable discrepancy between the absolute magnitude of vertical flux estimates derived from 14 C age-model MARs versus 230 Th normalized MARs. However, there is an emerging overall picture in the EEP of: (1) higher Marine Isotope Stage (MIS) 2 C org flux relative to the Holocene with a deglacial peak (Pedersen, 1983;Lyle, 1988;Kienast et al, 2007;Pichevin et al, 2009); (2) lower MIS 2 opal flux relative to the Holocene with a deglacial peak (Bradtmiller et al, 2006(Bradtmiller et al, , 2010Kienast et al, 2006;Warnock et al, 2007;Pichevin et al, 2009;Dubois et al, 2010;Calvo et al, 2011); and (3) constant or lower absolute MIS 2 CaCO 3 flux relative to the Holocene with a slight deglacial peak and better MIS 2 preservation (Loubere, 2004;Loubere and Richaud, 2007;Richaud et al, 2007;Mekik et al, 2012). High MIS 2 C org may be reconciled with low opal if Fe-fertilization decreased the diatom utilization ratio of Si:C and/or increased the solubility of frustules (Watson et al, 2000;Warnock et al, 2007;Pichevin et al, 2009;Dubois et al, 2010).…”

Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions

“…wind driven upwelling, nutrient content of water masses, biological competition for nutrients use) remain controversial and are likely related to changes in atmospheric and oceanic circulation. While several studies displayed millennial and/or glacial/interglacial changes of the opal production at low latitude in the eastern Pacific (Ganeshram and Pedersen, 1998;Kienast et al, 2006;Dubois et al, 2010;Pichevin et al, 2010;Arellano-Torres et al, 2011;Dubois et al, 2011;Romero et al, 2011a;Cartapanis et al, 2012), high temporal resolution opal records in northeastern Pacific are still limited.…”