Sulfur Isotopic Composition of Cenozoic Seawater Sulfate

Abstract

A continuous seawater sulfate sulfur isotope curve for the Cenozoic with a resolution of ϳ1 million years was generated using marine barite. The sulfur isotopic composition decreased from 19 to 17 per mil between 65 and 55 million years ago, increased abruptly from 17 to 22 per mil between 55 and 45 million years ago, remained nearly constant from 35 to ϳ2 million years ago, and has decreased by 0.8 per mil during the past 2 million years. A comparison between seawater sulfate and marine carbonate carbon isotope records reveals no clear systematic coupling between the sulfur and carbon cycles over one to several millions of years, indicating that changes in the burial rate of pyrite sulfur and organic carbon did not singularly control the atmospheric oxygen content over short time intervals in the Cenozoic. This finding has implications for the modeling of controls on atmospheric oxygen concentration.Global changes in climate and atmospheric chemistry are intimately related to the sedimentary S cycle (1, 2). Seawater sulfate represents one of the main reservoirs of dissolved S (3). Evidence for large-scale transfers of S between different sedimentary reservoirs is provided by the evaporite-based isotope record of oceanic sulfate (4 -6). Here, we present a high-resolution, continuous seawater S isotope age curve ( Fig. 1) that could help to elucidate the factors affecting the cycles of S and C and atmospheric oxygen content over geologic time.The concentration of dissolved sulfate in the ocean and its ␦ 34 S value are controlled by (i) a balance between the flux of S to the oceans from continental weathering and its isotopic value [␦ 34 S of this source is variable but is typically lower than seawater, 0 to 10 per mil (1)], and (ii) the deposition rate of S-bearing sediments, including evaporite sulfates with ␦ 34 S values approximately equal to that of seawater, and pyrite sulfide with a large isotopic fractionation signature [for example, modern ␦ 34 S values of pyrites average about -20 per mil (7)]. Volcanism and hydrothermal activity are also sources of reduced sulfur to the ocean [␦ 34 S values of these sources vary from about 0 to 3.5 per mil (1,8,9)]. Therefore, considerable S isotopic variations can be generated. For example, during periods of enhanced pyrite formation and burial, the light S isotope is preferentially removed from the ocean and the ␦ 34 S value of seawater sulfate increases. These and other fluctuations are recorded in contemporaneous evaporites and marine barite. Suitability of barite for recording seawater sulfate isotopic composition. Records of the S isotopic composition of seawater sulfate have been obtained mostly from isotope data on marine evaporite sulfates [for reviews see (5,6,10)], but the geological record of marine evaporites is episodic, with gaps of millions of years (5). Evaporites are also susceptible to diagenesis, their age control is often problematic (3), and a purely marine origin of some is questionable (11). In another attempt, Burdett et al. (12) used the S...

Down the Rabbit Hole: toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events

“…The long-term Cenozoic δ 13 C and δ 34 S records (Zachos et al, 2001;Paytan et al, 1998), when coupled together on the same time scale, strongly suggest that an immense amount of organic carbon without Fe-sulfide minerals accumulated in the late Paleocene (∼60-57 Ma), and a tremendous quantity of Fesulfides accumulated during the early Eocene (∼56-50 Ma). A huge mass of organic carbon placed into peat would provide a high C-low S reservoir, which then might have been "tapped" periodically to cause the PETM (Kurtz et al, 2003) and other negative carbon isotope excursions .…”

228Ra/226Ra and 226Ra/Ba ratios in the Western Mediterranean Sea: Barite formation and transport in the water column

“…Because barite forms in the water column, seawater elemental distribution patterns can be recorded by authigenic barite. For instance, sedimentary barite has been used to reconstruct the paleocomposition of seawater, including strontium ( 87 Sr/ 86 Sr), oxygen ( 18 O/ 16 O) and sulfur ( 34 S/ 32 S) isotopic compositions (Cecile et al, 1983;Paytan et al, 1993;Martin et al, 1995;Paytan et al, 1998). Attempts to reconstruct the past Nd isotopic composition of seawater ( 143 Nd/ 144 Nd) have also been made (Martin et al, 1995;Paytan, 1996).…”

Sulfur, Strontium, Carbon, and Oxygen Isotopes of Calcium Sulfate Deposits in Late Carboniferous Rocks of the Loei-Wang Saphung (LWS) Area, Loei Province, Thailand

“…Nevertheless, a comparison between the S and 87 Sr/ 86 S values' compositions of the marine evaporites with the values for contemporaneous seawater [22,26] revealed that most of the sulfate minerals analyzed from these deposits were simply derived from the precipitation of evaporated seawater.…”