Geology topics

T.K. Lowenstein

Publications and source records attributed to T.K. Lowenstein.

3 recordsLinked to original sources

USGS2005

An ostracode based paleolimnologic and paleohydrologic history of Death Valley: 200 to 0 ka

Death Valley, a complex tectonic and hydrologic basin, was cored from its lowest surface elevation to a depth of 186 m. The sediments range from bedded primary halite to black muds. Continental ostracodes found in the black muds indicate that those sediments were deposited in a variety of hydrologic settings ranging from deep, relatively fresh water to shallow saline lakes to spring discharge supported wetlands. The alkaline-enriched, calcium-depleted paleolake waters indicate extrabasinal streamflow and basin-margin spring discharge. The alkaline-depleted, calcium-enriched paleowetland waters indicate intrabasinal spring discharge. During Marine Isotope Stage 6 (MIS 6, ca. 180-140 ka) the hydrologic settings were highly variable, implying that complex relations existed between climate and basin hydrology. Termination II (MIS 6 to MIS 5E) was a complex multicyclic sequence of paleoenvironments, implying that climates oscillated between high and low effective moisture. MIS 4 (ca. 73-61 ka) was a spring discharge supported wetland complex. During MIS 2 (ca. 20-12 ka) the hydrologic settings were variable, although they are not fully understood because some black muds deposited during that time were lost during coring. ?? 2005 Geological Society of America.

Geological Society of America Bulletin↗

USGS2004

Seawater chemistry and the advent of biocalcification

Major ion compositions of primary fluid inclusions from terminal Proterozoic (ca. 544 Ma) and Early Cambrian (ca. 515 Ma) marine halites indicate that seawater Ca2+ concentrations increased approximately threefold during the Early Cambrian. The timing of this shift in seawater chemistry broadly coincides with the "Cambrian explosion," a brief drop in marine 87Sr/86Sr values, and an increase in tectonic activity, suggesting a link between the advent of biocalcification, hydrothermal mid-ocean-ridge brine production, and the composition of seawater. The Early Cambrian surge in oceanic [Ca2+] was likely the first such increase following the rise of metazoans and may have spurred evolutionary changes in marine biota. ?? 2004 Geological Society of America.

Geology↗

USGS2002

The major-ion composition of Silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg2+, Na+, and SO2-4, and much higher concentrations of Ca2+ relative to the ocean's present-day composition. Furthermore, Silurian seawater had Ca2+ in excess of SO2-4. Evaporation of Silurian seawater of the composition determined in this study produces KC1-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na+ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg2+/Ca2+ ratio of Silurian seawater was ~1.4, and the K+/Ca2+ ratio was ~0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg2+/Ca2+ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawater with Mg2+/Ca2+ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg2+/Ca2+ ratio of seawater, not the pCO2 of the Silurian atmosphere. Copyright ?? 2002 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗