Research Investigation 3, Task 3
Biosignatures and paleoenvironmental indicators in the Archean terrestrial record - Pilbara, Australia; Kaapvaal, South Africa; and Isua, Greenland

Core from the 2.5 Ga Kuruman Iron Formation, South Africa, showing fresh magnetite (Mt), siderite (Sid), and hematite-chert (Hem) layers. Core ~1 cm across.
Photo by PI Clark Johnson.
(Click on photo for large image.)

A substantial geologic and geochemical framework exists for terrestrial Archean sedimentary sequences, which have been at the center of debates on the origin and evolution of life on the early Earth, as well as its paleoenvironmental conditions, including the appearance of liquid surface water and an oxygen-bearing atmosphere. These studies, which are on-going and comprise major focal points of several current NAI groups, will provide an important springboard for our new C, O, S, Ca, Mg, and Fe isotope studies. Our work, where multiple isotopic systems will be applied to the same samples using both conventional and in situ analysis by ion microprobe, promises to provide new insights into the biological and paleo-environmental evolution of the early Earth. These studies will represent an important application of the biosignatures and paleoenvironmental indicators that will be developed in Investigation 2.

Our work in Task 3 will include:

• Investigate the iron sources and pathways in the earliest geological record for iron cycling in the 3.8 b.y. old banded iron formation at Isua, SW Greenland.In addition to Fe isotope studies, O and S isotope studies of oxides and sulfides, respectively, will constrain fluid sources and investigate possible mass-independent S isotope fractionations that may constrain atmospheric O² contents. Isotopic work will include in situ analysis using the new Cameca 1280 ion microprobe at UW Madison. Work at Isua will include assessment of the effects of metamorphism.
  Lead Investigator: Clark Johnson (UW-Madison)

• Study the origin of carbonaceous material in Early Archean cherts, Pilbara craton, Australia. Several ~3.5 b.y. old chert localities, including the Apex Chert and Marble Bar Chert, will be studied for C, O, S, and Fe isotope variations to address current debates on the origin of carbonaceous material that has been proposed to reflect early oxygenic photosynthesis on Earth. In addition to conventional isotopic work, in situ analysis will be made using the new Cameca 1280 ion microprobe at UW Madison.
  Lead Investigator: John Valley (UW-Madison)

• Determine the age and formation pathways of iron oxides in Early Archean paleosols, Pilbara craton, Australia. Proposed paleosols at Strelley Pool and North Shaw will be studied to determined the age of oxide formation through U-Pb geochronology and the sources of iron and temperature of formation of the oxides using O and Fe isotope geochemistry.
  Lead Investigator: Clark Johnson (UW-Madison)

• Constraining the timing of development of dissimilatory bacterial sulfate- and Fe³⁺-reduction through S and Fe isotope studies of iron oxides and pyrite from the Archean rock record. Development of these metabolisms depended on the existence of an oxidant on the surface of the Earth, although the nature of this oxidant is unclear.
  Lead Investigator: Clark Johnson (UW-Madison)

• Constrain the temporal record of biological and abiological iron cycling in banded iron formations of 3.5 to 2.5 b.y. age in the Kaapvaal craton (South Africa) and Pilbara craton (Australia). Work will include detailed mineralogical and chemical studies of diagenetic minerals (silica, oxides, carbonates), including conventional and in situ isotopic analysis of C, O, and Fe isotope variations.
  Lead Investigator: Clark Johnson (UW-Madison)

top of page