Archean sulfur reservoirs of the Kaapvaal Craton

Archean sulfur is recognized by anomalous isotopic signatures where D36S/D33S* = ~0.90. These signatures have been attributed to photolysis of SO2 in a low oxygen atmosphere and disappear from the rock concurrent with the timing of the Great Oxidation Event and the apparent rise in atmospheric oxygen. The main objective of this thesis is to examine the isotopic composition of sulfide minerals from different depositional environments and explore the mechanisms which preserve the anomalous signatures. The South African Witwatersrand and Pongola Supergroups are the primary study areas. Both are Mesoarchean in age and contain well-preserved sedimentary sequences. They were chosen as they contain a wide variety of lithostratigraphic occurrences of pyrite; here samples were collected from diamictites, paleosols and fluvial and marine clastic sedimentary rocks. The texture and morphology of sulfide grains were documented via reflected light microscopy, etching and backscattered electron imaging. This was combined with LA-ICP-MS trace element measurements to determine paragenesis. The sulfur isotope composition was defined through in-situ four isotope analysis using the SHRIMP-SI. Pyrite from diamictites returned mostly negative D33S and D36S values, in the ranges of -0.43 - 0.09 per mil (pm) and -1.95 - 0.15 pm, respectively. The d34S** values showed a range of -6.09 - 11.7 pm. Deviation in D36S from the atmospheric D36S/D33S array and variation in d34S is consistent with microbial sulphate reduction of atmospheric sulphate. The restricted d34S values observed in one sample set may represent input of mass-dependent crustal sulfur. Paleosol-hosted sulfide minerals had negative or near-zero D33S values, with a range of -0.50 - 0.17 pm, and near-zero D36S values, with a range of -0.99 - 0.52 pm. The d34S values showed a range of -8.88 - 6.87 pm. In samples where both D33S and D36S were ~0 pm, the sulfur is likely magmatic sulfur inherited from the igneous parent rocks. Samples with small negative D33S values likely preserve a mixture of atmospheric sulfate and inherited magmatic sulfur. Pyrite from fluvial rocks typically returned negative D33S and D36S values, with the majority of data falling in the ranges of -0.72 - 0.10 pm for D33S and -0.98 - 0.34 pm for D36S. The d34S values typically fell within the range of -3.21 - 9.65 pm. The muted negative D33S values and small d34S values are consistent with a mixture of atmospheric sulfate and mass-dependent crustal sulfur. The consist deviation in D36S from the atmospheric D36S/D33S array suggests microbial sulfate reduction. Shallow marine samples returned variable isotopic signatures. Several samples showed D36S/D33S = ~6.9, consistent with microbial sulfate reduction. These samples showed D33S values of -0.18 - 0.51 pm, D36S values of -2.05 - 0.02 pm and d34S values of -15.8 - 8.53 pm. Other samples showed D36S/D33S = ~-2, with D33S values of 0.11 - 0.80 pm and D36S values of -2.04 - -0.25 pm. The d34S values showed a range of -1.17 - 7.74 pm. Data from individual samples were observed to show consistent D33S but variable D36S, forming steep arrays extending downwards from the atmospheric D36S/D33S array. This was interpreted to represent microbial fractionation of atmospheric elemental sulfur. These results indicate that terrestrial and marine depositional environments preserve different isotopic signatures. Terrestrial environments favour preservation of atmospheric sulfate and marine environments favour preservation of atmospheric elemental sulfur. Within both environments, microbial processes were involved in preservation of the atmospheric sulfur species. Atmospheric signatures in terrestrial environments are muted, which is attributed to mixing of atmospheric and crustal sulfur. Signatures are also muted in marine samples, which may also be due to mixing, or possibly some other localized environmental effect. *D = Capital delta **d = lowercase delt

A multiple sulfur record of super-large volcanic eruptions in Archaean pyrite nodules

Archaean supracrustal rocks carry a record of mass-independently fractionated S that is interpreted to be derived from UV-induced photochemical reactions in an oxygen-deficient atmosphere. Experiments with photochemical reactions of SO2 gas have provided some insight into these processes. However, reconciling experimental results with the multiple S isotopic composition of the Archaean sedimentary record has proven difficult and represents one of the outstanding issues in understanding the Archaean surface S-cycle. We present quadruple S isotope data (32S, 33S, 34S, 36S) for pyrite from Mesoarchaean carbonaceous sediments of the Dominion Group, South Africa, deposited in an acidic volcanic lake, which help reconcile observations from the Archaean sedimentary record with the results of photochemical experiments. The data, which show low S/S ratios (mostly ≪ 1) and very negative S/S ratios (−4 and lower), contrast with the composition of most Archaean sedimentary sulfides and sulfates, having S/ (the so-called ‘Archaean reference array’), but match those of modern photochemical sulfate aerosols produced in the stratosphere, following super-large volcanic eruptions, and preserved in Antarctic ice. These data are also consistent with the results of UV-irradiation experiments of SO2 gas at variable gas pressure. The S isotope composition of the Dominion Group pyrite is here interpreted to reflect the products of photolysis in a low-oxygen-level atmosphere at high SO2 pressure during large volcanic eruptions, mixed with Archaean ‘background’ (having a composition broadly similar to the Archaean reference array) S pools. It is inferred that high sedimentation rates in a terrestrial basin resulted in an instantaneously trapped input of atmospheric S during short-lasted depositional intervals, which faithfully represents transient photochemical signals in comparison with marine sedimentary records