DOI: 10.25921/9jy3-bq31
title: NOAA/WDS Paleoclimatology - Eastern Equatorial Pacific 100KYr Opal Flux Data

description: 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 CO2. 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 (231Pa/230Th)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.
description: The data set contains compositional data (opal, CaCO3, 
Corg, lithogenic) and 230Th-normalized fluxes from 6 cores 
from the eastern equatorial Pacific covering the late 
Quaternary, in addition to relative changes in Brassicasterol 
content in core ME0005A-24JC. 

The age models for cores ME0005A-24JC, TR163-19P, TR163-22P, 
TR163-31P and V19-30 are adopted as previously published: 
ME0005A-24JC (Lyle et al., 2005; A. Mix, unpublished data, 2006);
TR163-19P (Kienast et al., 2006); TR163-22P (Lea et al., 2006);
TR163-31P (Kienast et al., 2006); V19-30 (Shackleton et al., 1983). 

The age model for core ME0005A-27JC is adopted as published 
(Kienast et al., 2007) for the last 35 kyr, while the oldest 
part is based on benthic d18O stratigraphy 
(A. Mix, unpublished data, 2006).

Biogenic opal was determined colorimetrically following 
alkaline extraction of silica (Mortlock and Froelich, 1989). 
Biogenic opal values in cores TR163-19P and TR163-31P are 
taken from Kienast et al. (2006) and those of core V19-30 
for the last 20 kyr B.P. are taken from Bradtmiller et al. (2006). 

Uranium (238U), thorium (232Th, 230Th) and protactinium (231Pa) 
concentrations were determined at the University of British 
Columbia (UBC), LDEO and the Woods Hole Oceanographic Institution 
(WHOI) by isotope dilution on an inductively coupled plasma mass 
spectrometer (ICP-MS) following total acid digestion of sediment 
samples equilibrated with 229Th and 236U spikes and anion resin 
column chemistry to separate the Pa and U/Th fraction 
(François et al., 2004). In the calculations of 230Thxs,0 
and 231Paxs,0 we use a half life for 230Th of 75.69 kyr, 
a half life for 231Pa of 32.50 kyr and a A238Udet/A232Thdet 
ratio of 0.50. Radioisotope data for cores TR163-19P and TR163-31P 
are from Kienast et al. (2006), and those of core V19-30 for 
the last 20 kyr are from Bradtmiller et al. (2006). 
Data for the last 35 kyr B.P. for cores ME0005A-24JC 
and ME0005A-27JC are from Kienast et al. (2007). 

Carbonate (CaCO3) values were obtained from the coulometric 
CO2 determinations assuming no other carbonate-bearing phase 
was present. 

Total carbon was determined by flash combustion gas chromatography 
with CHN elemental analyzers.

Organic carbon was estimated by subtracting carbonate from 
total carbon.

The lithogenic fraction was estimated from the residual 
non-biogenic fraction of the sediment, after subtraction of opal, 
carbonate and organic matter, except for core V19-30, 
where it was calculated from 232Th, assuming a detrital 
source of all 232Th and an average 232Th dust content 
of 10 ppm (Taylor and McLennan, 1985).

Changes in the relative concentration of brassicasterol 
in core ME0005A-24JC were determined by gas chromatography 
at Dalhousie University. Approximately 1 g of freeze-dried 
sediment was extracted using a Dionex Accelerated Solvent 
Extraction system (ASE200). These extracts were saponified 
using potassium hydroxide and purified through silica column 
chromatography. The purified extracts were then treated 
with bis(trimethylsilyl)trifluoroacetamide (BSTFA) to derive 
trimethyl silyl esters (TMS) before gas chromatographic 
analysis. Relative sterol distributions were determined 
by integration of the sterol peak area in the chromatogram, 
normalized to sample weight. We did not use recovery 
standards and thus the normalized brassicasterol 
chromatographic peak areas we report were not corrected 
for potential losses during the extraction process.