D33S EVIDENCE FROM SUPERIOR PROVINCE ORE DEPOSITS FOR A SULFATE-STRATIFIED OCEAN AT 2.7 GA

We measured the multiple sulfur isotopic composition (³²S, ³³S, ³⁴S) of separated sulfides from ~2.7 Ga volcanic massive sulfide deposits and iron formations of the Archean Superior Province of central Canada.  Analyzed sulfides exhibit d³⁴S values between -5 to 5 ‰ and D³³S measurements of the same grains are all £ 0 ‰.  The data show a first-order correlation between the D³³S of sulfides from a given ore deposit and the classification of that deposit as a shallower water (D³³S £ 0 ‰) or deeper water (D³³S ~ 0 ‰) deposit.

The precipitation and successive dissolution of sulfate minerals in modern oceanic hydrothermal systems leads to the preservation of an oceanic d³⁴S signature in sulfides from sites of seafloor mineralization.  The d³⁴S systematics of sulfides in Archean submarine ore deposits can largely be explained as a combination of a similar process and the leaching of S from volcanic sulfides in the footwall rocks of the deposit.  Both photochemical experiments and measurements of ca. 3 Ga sulfate minerals suggest the D³³S of the water-soluble (i.e., SO₄) component of the sulfur cycle was uniformly negative.  The presence of a negative ³³S anomaly in ore sulfides from shallower water deposits, therefore, explicitly indicts seawater SO­­₄ reduction (either directly or through intermediary sulfate minerals) as a primary process in their formation.  The absence of a significant ³³S anomaly in ore sulfides from deeper water deposits, on the other hand, indicates that: 1) seawater SO₄ was not present in the ore-forming hydrothermal fluids; or 2) seawater SO₄ was present but was not reduced during the formation of the ore.  The first interpretation is more likely both on analytical grounds and in terms of the paleohydrology of the deeper water deposits.

A lack of SO₄ reduction during the formation of the deeper water deposits is strong evidence that a SO₄-stratified ocean was in place at ~2.7 Ga and suggests a limited sulfate source to the Archean surface ocean.  The hypothesis of a SO₄ gradient in the Archean ocean driven by a weak and possibly photochemical SO₄ source implies a contemporaneous low-O­₂ atmosphere and is fully testable with D³³S measurements from samples that show a similar correlation with depth.