3.5 GA DEEP-WATER FOSSIL HYDROTHERMAL FIELD AND SUBAERIAL DIAMICTITE ─ HOT ARCHEAN CRUST IN DEEP COOL OCEANS

Speculations about climate on our young Earth are based on models that trade-off elevated concentrations of greenhouse gasses against lower solar radiation from a fainter sun; as well as on geological observations from the rock and mineral record that imply surface water on our planet as far back as 3.8 Ga, and possibly 4.3-4.4 Ga. The temperature range of its oceans in the early Archean (ca. 3.5 Ga) is in dispute ─ with estimates between 40^o to 80^oC, primarily based on stable isotope proxies measured on cherts. However, the values of these proxies can be interpreted differently as either reflecting hot seawater or hydrothermal fluids, and climate reconstructions that far back are therefore highly controversial.

We present new field observations integrated with oxygen isotope and geochemical data on detailed stratigraphic sections of volcanic rocks and cherts from the Barberton Greenstone Belt, South Africa. We describe a 3.47 Ga fossil hydrothermal field containing silica pipes that formed at about 200^oC on seafloor ca. 2 km below sea-level. The pipes were subsequently buried beneath a 2 km thick pile of lavas during 5 myrs of discontinuous hydrothermal activity at successively lower temperatures, whilst the ocean floor cooled and deepened to ca. 4 km. Thereafter the ocean floor was tectonically uplifted and rapidly emerged above sea-level and we demonstrate that by 3.46 Ga, island arc-related hydrothermal systems dominated and continued until 3.3 Ga at temperatures ranging between 40^o to 270^oC in water depths less than 70 m and in places operated sub-aerially.

In concert, two independent field observations on associated subaerial and deep water sedimentary rocks formed at relative low latitudes (20-40^o, as determined from paleomagnetism) suggest that the temperatures of the atmosphere and deep oceans were relatively cool: the first concerns 3.43-3.45 Ga diamictites and local drop-stones resembling modern glacial deposits; the second concerns authigenic-like gypsum formed within 3.72 Ga abyssal muds, as they do in present day deep sea sediments affected by cold-water bottom currents. Our results suggest therefore that Paleoarchean surface temperatures and climates were comparable with those of more recent times.