THE IMPORTANCE OF SALINITY ANALYSIS IN PALEOENVIRONMENTAL STUDIES

Recently refined elemental proxies for paleosalinity determinations in bulk shale samples offer new windows into depositional conditions in ancient cratonic and marginal-marine basins. The most promising of these proxies, boron/gallium (B/Ga), can robustly distinguish between marine (>6), brackish (3-6), and freshwater (<3) conditions, although additional proxies (Sr/Ba and S/TOC) can provide supporting evidence. Sediment B/Ga appears to be particularly sensitive to paleosalinity fluctuations (owing to the strong correlation of aqueous B to salinity) as well as resistant to secondary alteration of primary signals (owing to the general tightness of shales in the burial environment).

In this presentation, B/Ga and supporting data will be examined for four ancient cratonic depositional systems, all of which show at least transient development of brackish or freshwater conditions in facies that have long been regarded as fully marine. In the Late Pennsylvanian Midcontinent Sea of North America, salinities varied in both the geographic and stratigraphic dimensions in a manner consistent with control by the major orbital-scale glacio-eustatic fluctuations that produced cyclothems. In the Early Jurassic Cleveland Basin of the U.K., salinities fell sharply to low-brackish conditions (~1-10 psu) during the Toarcian oceanic anoxic event, before recovering to background high-brackish values (~10-30 psu). In the Late Devonian Illinois Basin of North America, marine salinities during the Frasnian yielded abruptly to low-brackish salinities during the Famennian, triggering a shift from red algae to green algae as the dominant primary producers. In the Neuquen Basin of Argentina, Triassic-Jurassic boundary strata yielded the greatest surprise of all—units that had long been thought to be marine based on the presence of a few marine fossils turned out to be freshwater or nearly so. We infer that the Neuquen Basin must have been fully separated from the Panthalassic Ocean to the west by a narrow range of coastal mountains, a situation analogous to San Francisco Bay in modern California. These examples demonstrate that correct interpretation of depositional conditions and controls on watermass redox and productivity patterns in cratonic and marginal-marine systems is not possible without paleosalinity data.