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Publication Title | Pore Fluid Constraints on Deep Ocean Temperature and Salinity during the Last Glacial Maximum

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*

Lamont-Doherty Earth Observatory, Palisades, NY 10965

Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street,

Cambridge, MA 02138

Now at: California Institute of Technology, 1200 California Ave, MS 100-23,

Pasadena, CA, 91125

Abstract. Pore water records of δ18O and [Cl] from ODP Site 1063A on the Bermuda Rise constrain the change in seawater δ18O and salinity from the Last Glacial Maximum (LGM) to the Holocene to be 0.75±0.05‰ and 2.5±0.1% respectively. Coupled with a measured benthic foraminiferal δ18O change, this result means that bottom waters were 4.6±0.8°C cooler than the Holocene at the LGM and therefore at or near the seawater freezing point. Coupled δ18O and chlorinity results give an extrapolated mean ocean LGM to Holocene change in δ18O of 0.95±0.09‰. These data also constrain the past southern source deep-water salinity to be 35.76±0.04 psu, which is within error of the mean deep ocean value for this time.

1. Introduction

A direct approach to measuring LGM seawater δ18O

is based on the isotopic composition of deep-sea pore

fluids. The isotopic history of the bottom water over

glacial cycles provides an oscillating boundary condition

on the sediment-pore fluid system yielding a record of

past ocean composition attenuated by diffusion. This

approach is similar to the method used to reconstruct

surface temperatures from borehole temperature profiles

in ice cores [Cuffey et al., 1995]. McDuff [McDuff, 1985]

first recognized the signal of Pleistocene glacial cycles in

deep sea pore fluids from Deep Sea Drilling Project

(DSDP) Site 576 in the central Pacific. Schrag and

DePaolo [Schrag and DePaolo, 1993], using data from

McDuff, showed that the pore fluid profile was primarily

a measure of the change since the LGM, and that one

could obtain an estimate for δ18O during the LGM by sw

using a numerical model to calculate the effects of diffusion. They calculated a change of 1.0±0.25‰, although the error was limited by low sampling resolution and analytical precision. Schrag et al. [Schrag et al., 1996] measured a pore fluid profile from a site at 3000m on the Ceara Rise with higher sampling resolution (every 1.5 m) and better precision (±0.03 ‰), obtaining a value for δ18Osw of 0.8±0.1‰, substantially lower than the previous estimates. This value is a combination of the global effect of sea-level change and a local effect due to changing deep-water masses at the site. The local circulation effect is most significant for the deep North Atlantic basin which experienced a change from relatively low-δ18O, southern source deep water during the LGM to

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relatively high-δ O, northern source deep water in the

Holocene. Given this complication, Schrag et al. [1996] suggested that the global average value (i.e., due to ice volume only) was 1.0±0.1‰, based on two different methods of extrapolation.

The pore water diffusion approach will work for any chemical or isotopic tracer that is not affected substantially by chemical reactions with sediments over the time scale of glacial cycles (i.e., conservative). Outside of methane clatherate chemistry (see Results), chlorinity is a conservative tracer (unlike salinity) and can be used to reconstruct the salinity of LGM seawater if precise measurements can be obtained. McDuff [McDuff, 1985] also identified a change in chlorinity in the pore fluids from DSDP Site 576, that correlated with the change in δ18O, although the data showed scatter due to low precision and sampling resolution. In this paper we present better measurements of chloride from deep-sea pore fluids from the Bermuda Rise, combined with high- precision δ18O data. We show that by combining both data sets, we can determine the local change in seawater

The oxygen isotopic composition (δ18O) [Epstein et al., 1953] of benthic foraminiferal CaCO is a function of

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both the temperature and the δ18O of the water in which

the foraminifera grows. The δ18O of seawater (δ18O ) is sw

related to the amount of isotopically-depleted ice that is stored on land during glaciations. This ambiguity in carbonate values has led to the interpretation of foraminiferal δ18O records as mostly due to changes in temperature [Emiliani, 1955] and alternatively as mostly due to variations in ice volume [Shackleton, 1967].

Several approaches have been taken to determine how much each of the two factors, temperature and ice volume, contribute to the total change in foraminiferal δ18O. Fairbanks and Matthews [1978] measured the

change in δ18O of seawater per meter of sea level change

in uplifted Barbados corals that grew during past sea level

high stands. They calculated a maximum value of

0.011‰/meter which, when combined with the measured

change in sea level since the LGM (121±5 m) [Fairbanks,

1989], yields an estimate of glacial-interglacial change in

δ18O of 1.3‰. Chappell and Shackleton [1986] used a sw

similar approach at the Huon Peninsula, obtaining a

slightly lower value. Both these studies assume that

temperature change is constant at their sites during the

period when sea level is changing, an interpretation later

contradicted by Guilderson et al. [1994]. Thus, their

estimates are maximum values depending on the

magnitude of tropical cooling during the LGM. An

additional complication is that there is no requiremenmt

that the change in δ18O at a single location is sw

representative of the global oceam. This is especially important in the surface ocean where evaporation and

precipitation can lead to large local changes in δ18O . sw

Adkins and Schrag 3/11/2000 4:24 PM

Pore Fluid Constraints on Deep Ocean Temperature and Salinity during the Last Glacial Maximum

1,*. 2 J. F. Adkins and D. P.Schrag

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