David E. Krantz

Profiles of strontium and barium in Mercenaria mercenaria and Spisula solidissima shells

Abstract Cross sections of two Mercenaria mercenaria shells, one modern and one Pleistocene, and one modern Spisula solidissima shell were analyzed for trace metals using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These data are coupled with 18 O 16 O ratio profiles, which are used as a temperature proxy to determine the season of growth for a given shell parcel. The shells show Sr and Ba fluctuations in regular annual patterns. Seasonal Sr decreases associated with the winter dormant period in Mercenaria , and with the summer growth decrease in Spisula indicate that kinetic control (growth rate) plays a more important role than temperature in the incorporation of this element. Barium exhibits sharp increases that are likely linked to periods of intense phytoplankton productivity. This application of LA-ICP-MS to the analysis of mollusk shells for documenting ontogenetic trends demonstrates the power of this technique for extracting geochemical information.

A chronology of Pliocene sea-level fluctuations: The U.S. Middle Atlantic Coastal Plain record

Abstract The sequence of marine and marginal-marine silts and sands of the U.S. Middle Atlantic Coastal Plain preserves a relatively complete record of Pliocene sea-level highstands. A general chronology for the deposition of these units has been developed previously from studies of the microfossil biostratigraphy complemented by a limited number of paleomagnetic and radiometric dates. Within this broad temporal framework it is currently difficult to assign ages to individual transgressive events with a resolution much better than 0.5 Ma. The age model proposed here attempts to refine the depositional history of the Atlantic Coastal Plain by correlation with the more continuous deep-ocean record. The model assigns probable ages to the Pliocene transgressions onto the Atlantic Coastal Plain based on evidence for sea-level highstands inferred from high-resolution deep-ocean δ 18 O records. At least five major transgressive-regressive events can be correlated with presumed eustatic fluctuations. The temporal resolution for the timing of the transgressive events is greatly improved over that of the biostratigraphy alone. Many aspects of the proposed chronology require verification by more detailed field study, but it is offered as a testable working hypothesis.

Oxygen isotopic identity of the Delaware Coastal Current

Results are presented and discussed from a preliminary study on the oxygen isotopic composition of coastal and inner shelf waters from the Middle Atlantic Bight between the mouths of Delaware Bay and Chesapeake Bay. The water samples were collected during the spring, April/May 1993, in conjunction with a detailed hydrographic survey of the Delaware Coastal Current. The relationship between salinity and δ18O, both of which are generally accepted conservative properties of seawater, allows identification of the fresh and saline mixing end-members for the Delaware Coastal Current. The main source of freshwater to the Delaware Coastal Current is a mixture of runoff into Delaware Bay with an estimated δ18OS = 0 composition of −7.08‰. The freshwater input is dominated by Delaware River discharge with a mean δ18O of −8.0‰ based on previous studies; there appears to be a secondary contribution of freshwater that is isotopically heavier from coastal-plain drainage. The extrapolated δ18OS = 32.35 value (−0.93‰) of the saline component implies exchange with Middle Atlantic Bight Shelf Water through along-shelf and across-shelf mixing processes. Limited data suggest that the oxygen isotopic composition of low-salinity water exiting the Chesapeake Bay mouth is discernible compared with that of the Delaware Coastal Current.

Ecostratigraphic Datums and Sequence Stratigraphy: Application to the Marine Quaternary

The marine Quaternary is characterized by few evolutionary appearances and extinctions of planktonic foraminifera. Because climatic fluctuations are a fundamental characteristic of Pleistocene, however, better stratigraphic resolution of the marine Quaternary can be gained by the establishment of biozones based on climatically controlled foraminiferal assemblages. Utilizing relative abundances of the warm-water Globorotalia menardii complex and temperature-water G. inflata, supplemented by left- and right-coiling varieties of G. truncatulinoides, the authors have subdivided the prezone-W Pleistocene of the tropical Atlantic (Core V16-205), Caribbean Sea (DSDP Core 502B), and northeast Gulf of Mexico (ODP Core 625B, Eureka Core E67-135) into 17 subzones, each with an average duration of {approximately}100,000 yrs. The subzones appear to reflect water mass shifts and disjunct species distributions resulting from expansion and contraction of northern hemisphere ice sheets. Hence, subzonal boundaries should also reflect change in eustatic sea level and sequence boundaries. Indeed, graphic correlation of the subzones, along with biostratigraphic markers and paleomagnetic and oxygen isotope datums, reveals changes in sediment accumulation rate (especially on the continental slope) and missing section, as well as intervals of deformation (core breaks) that affect the occurrence of subzonal boundaries and biostratigraphic markers.