Enriqueta Barrera

Evidence for thermohaline-circulation reversals controlled by sea-level change in the latest Cretaceous

Fluctuations in oxygen (δ 18 O) and carbon (δ 13 C) isotope values of benthic foraminiferal calcite from the tropical Pacific and Southern Oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m.y. interval (71‐70 Ma) in the Maastrichtian. At the onset of this change, benthic foraminiferal δ 18 O values increased and were highest in low-latitude Pacific Ocean waters, whereas benthic and planktic foraminiferal δ 13 C values decreased and benthic values were lowest in the Southern Ocean. Subsequently, benthic foraminiferal δ 18 O values in the Indo-Pacific decreased, and benthic and planktic δ 13 C values increased globally. These isotopic patterns suggest that cool intermediate-depth waters, derived from high-latitude regions, penetrated temporarily to the tropics. The low benthic δ 13 C values at the Southern Ocean sites, however, suggest that these cool waters may have been derived from high northern rather than high southern latitudes. Correlation with eustatic sea-level curves suggests that sea-level change was the most likely mechanism to change the circulation and/or source(s) of intermediate-depth waters. We thus propose that oceanic circulation during the latest Cretaceous was vigorous and that competing sources of intermediate- and deep-water formation, linked to changes in climate and sea level, may have alternated in importance.

Does ice drive early Maastrichtian eustasy

A large (30–40 m), rapid (≪1 m.y.), earliest Maastrichtian sea-level drop inferred from New Jersey sequence stratigraphic records correlates with synchronous δ18O increases in deep-water benthic and low-latitude surface-dwelling planktonic foraminifera. The coincidence of these events argues for the development of a moderate-sized ice sheet during the early Maastrichtian.

Thermal potentiation and mineralogical evolution in the Bivalvia (Mollusca)

The most important factor controlling the timing of Phanerozoic mineralogical evolution in the Bivalvia appears to be thermal potentiation of calcite deposition in colder marine and estuarine environments. Cold temperature has promoted mineralogical evolution in the Bivalvia by kinetically facilitating (potentiating) initially weak biological controls for calcite, thereby exposing their genetic basis to natural selection. Calcite has evolved in bivalve shells for a variety of selective advantages, including resistance to dissolution; resistance to chemical boring by algae and gastropods; reduced shell density in swimming and soft-bottom reclining species; enhanced flexibility in simple prismatic shell layers; and fracture localization and economy of secretion in association with certain foliated structures. Endogenous calcite in bivalve shells varies from biologically induced to weakly and strongly biologically controlled. Biologically controlled calcite generally first appears in bivalve shells as an impersistent component of the outer shell layer, only later, in some groups, expanding to include the entire outer and then part or all of the middle and inner shell layers. The initial stages of mineralogical evolution are shown by certain modern Mytilidae, Veneridae and Petricolidae. In the latter two families, the calcite occurs as conellae in the outer part of the outer shell layer. Calcitic conellae in the inner shell layer of Pliocene Mercenaria are not barnacle plates, as previously indicated, but endogenous calcite comparable in origin to other venerid conellae. Their occurrence in Mercenaria may reflect thermal potentiation of weak biological controls for calcite, as well as local detachment of the secretory mantle epithelium near the pallial and adductor musculature.

Seasonal Variation in Oxygen Isotopic Composition of Two Freshwater Bivalves: Sphaerium striatinum and Anodonta grandis

Oxygen isotopic values have been obtained from microsamples of the aragonitic freshwater bivalves Sphaerium striatinum (Pisidiidae) and Anodonta grandis (Unionidae) collected alive from Wellington Creek, OH. To test whether these organisms secrete their shell in isotopic equilibrium, the SO values of shell aragonite are compared to ambient water temperature and δ18O values monitored for > 1 yr. These bivalves were chosen for study because they are abundant in surface sediments and cores from Lake Erie where they represent a source of information on the environmental history of the lake. The observed mean values are −5.54‰ for A. grandis and −6.16‰ for S. striatinum. The mean δ18O value expected for bivalve aragonite if equilibrium precipitation is occurring during May–August in Wellington Creek is −5.69‰. The similarity between measured and predicted isotopic values for both species suggests that they are useful sources of paleoenvironmental data. Overall, the isotopic composition of the shells of the two species reflects less than one half of the calculated range of potential biogenic aragonite values for the stream and omits recording evaporative conditions associated with ponded water. Bivalve δ18O and δ13C data covary. The δ13C data are highly negative and values could reflect 12C enrichment of dissolved organic carbon from organic matter oxidation and/or ingestion of food carbon.

Middle Holocene Hydrologic Change and Hypolimnion Formation in Lake Erie

Data from a nearshore sediment core and a deep-water sediment core from the central basin of Lake Erie reveal shifts in sediment properties and stable isotope composition of shell carbonate between ca. 4,600 and 3,500 14C yrs BP. Radiocarbon dates are corrected for the hardwater effect by subtracting 670 years based on a modern calibration for the central basin. Silt content increased in the deep water core at 4,600 and again, slightly, at 3,900 14C yrs BP. Sand size increased in the nearshore core at 3,500 14C yrs BP. δ18O of shell carbonate increased and δ13C decreased in both cores between about 4,200 and 3,500 14C years BP. Magnetic susceptibility and percent calcite decreased sharply and percent organic carbon increased slightly in the deep water core beginning at 4,000 14C yrs BP. Most of the changes in sediment properties and stable isotope composition of shell carbonate occurred between 4,200 and 3,900 14C yrs BP. They coincide in time with the Nipissing II highstand of Lake Nipissing, in the Huron and Michigan basins, and with evidence for higher-than-present lake levels in Lake Erie. The changes in proxy data are interpreted as evidence for an influx of surface water as drainage from the Upper Great Lakes was rerouted through Lake Erie. There is little evidence in the sediment proxy record for changes in Lake Erie during the earlier Nipissing I highstand or the middle Holocene transition in regional climate. A 9,000-year composite stable isotope record for the central basin shows that the sediment cores document a transformation in drainage that established the modern hydrologic system for the lake. High lake level induced a seasonal hypolimnion, setting the stage for the low pH, oxygen-depleted bottom waters of today.