Dorothea Ames

Sr and Nd isotopic analysis of phosphorite sedimentation through one Miocene high-frequency depositional cycle on the North Carolina continental shelf

Abstract A well-developed seismic-, litho-, and biostratigraphic data base with good models for the formation and deposition of phosphate-rich sediments exist for the Upper Cenozoic sediments on the North Carolina continental margin. This provides a unique opportunity to evaluate the potential of utilizing strontium and neodymium isotopic analyses to help unravel a complex genetic history of phosphate formation and deposition. Thirty one samples from three vibracores in Onslow Bay supplied hand-picked concentrate subsamples of five phosphate grain types and thirty one concentrate subsamples of phosphate peloids for stratigraphic analysis. All subsamples were analyzed for their Sr isotope composition, while only thirteen were analyzed for Nd isotopes. The 31 peloid samples represent three different stratigraphic units and include: (1) Holocene surficial sands with highly variable concentrations and types of reworked phosphate; (2) Pleistocene moldic carbonates with minor concentrations of black peloidal phosphate; and (3) Miocene (mid-Burdigalian), Frying Pan Sequence unit 1 (FPS-1) of the Pungo River Formation with very high concentrations of multiple types of phosphate grains that decrease upsection and change grain types with changing lithofacies. The latter unit represents deposition during one high-frequency sea-level cycle. Stratigraphic analysis utilized one grain type, phosphate peloids, for comparative purposes with the depositional model of Riggs and Mallette (1990). The Sr and Nd data cluster into significant categories that are coincident with these three major stratigraphic units. Sr and Nd isotope compositions of selected phosphate grains from three phosphate-rich depositional sequences are utilized to: (1) date the initial Miocene transgression of unit FPS-1 of the Pungo River Formation; (2) decipher processes of formation and deposition of major phosphate grain types through one Miocene sea-level cycle including the processes of in situ formation versus reworking; (3) develop a Sr and Nd chronostratigraphy for evaluating changing patterns of continental margin sedimentation through high-frequency depositional cycles; (4) determine effects of reworking and weathering on the isotope signals between Miocene, Pleistocene, and Holocene sediments; (5) evaluate the suitability of utilizing Sr and Nd isotopes for detailed chronostratigraphic analysis of phosphorite sedimentation; and (6) refine our understanding of the Upper Cenozoic evolutionary history of North Carolinas continental margin.

Foraminiferal and Sedimentary Record of Late Holocene Barrier Island Evolution, Pea Island, North Carolina: The Role of Storm Overwash, Inlet Processes, and Anthropogenic Modification

Abstract Foraminiferal and sedimentary data, supplemented with geochemical dating and ground-penetrating radar transects, show that the barrier island at Pea Island National Wildlife Refuge just north of Rodanthe, North Carolina, has been dominated by a combination of inlet and overwash processes for at least 1000 years. The stratigraphic record of several vibracores does not preserve every, or even many, overwash events but, instead, is characterized by three to four fining-upward sequences. The last three commence with overwash sand or gravel that is overlain by a variety of finer-grained estuarine, inlet, and marsh deposits. The dynamic nature of this segment of the Outer Banks was muted in the late 1930s by construction of artificial barrier dune ridges, extensive planting of grass and shrubs, and construction of Highway 12 in 1953. Subsequently, the road and barrier dune ridge were rebuilt and relocated several times following storm events.

Geospatial Analysis of Barrier Island Width of Two Segments of the Outer Banks, North Carolina, USA: Anthropogenic Curtailment of Natural Self-Sustaining Processes

Abstract A comparison of two sections of the Outer Banks, North Carolina, USA (Pea Island and Avon-Buxton areas), reveals the importance of the interplay between oceanic and estuarine shoreline dynamics to long-term changes in barrier island width. From 1852 to 1998, the northern portion of Pea Island experienced an average net increase in width of 431 m (3 m/y); this area experienced low to moderate rates of oceanic shoreline erosion and high rates of back-barrier land accretion via overwash and formation of flood tidal delta islands. In contrast, between 1852 and 1998, the width of the southern portion of Pea Island and the Avon-Buxton area decreased an average of 515 m (4 m/y) and 594 m (4 m/y), respectively, because of high rates of oceanic shoreline erosion and variable changes in estuarine shoreline accretion and erosion. Net gain or net loss of barrier island width is strongly dependent on the natural depositional processes of overwash and flood tide delta formation. Anthropogenic modifications to the barrier island, such as construction of barrier dune ridges, planting of stabilizing vegetation, and urban development, can curtail or even eliminate the natural, self-sustaining processes of overwash and inlet dynamics.

Sr Isotopic Age Analysis of Co-Occurring Miocene Phosphate Grain Types on the North Carolina Continental Shelf

ABSTRACT Four phosphate grain types and associated foraminifera were analyzed for their 87Sr/86Sr ratios to test the hypothesis that different authigenic grain types have specific times and processes of formation in response to changing conditions during one sea-level transgression. The samples were collected from one phosphate-rich depositional sequence (FPS-1) of the Miocene Pungo River Formation in Onslow Bay, North Carolina continental shelf. The hand-picked grains were from three cores representing upsection changes from lowstand, to transgressive, and highstand systems tracts. The phosphate grain types carried significantly different strontium isotopic signatures and included the following: peloidal grains (spherical to oval, highly rounded and polished grains with inor inclusions and common coatings), intraclastic grains (irregular-shaped rip-up grains from preexisting crusts and nodules), microsphorite (laminated crusts and bored hardgrounds associated with unconformities and containing common inclusions), and skeletal grains (formed directly by organisms). Four different phosphate skeletal grains were analyzed (brachiopods, vertebrae, teeth, and bone splinters); however, the results were not significantly different and therefore they were all included as one grain type. Analyses of phosphate grain types and associated foraminifera support the interpretation that they formed in response to specific chemical and physical processes and at different times during phosphogenesis, as follows. (1) Peloids cluster tightly and have Sr isotope ages contemporaneous with the associated foraminifera. They are interpreted to represent the time of phosphogenesis and primary sedimentation during early- to mid-stage transgression. (2) Intraclasts are significantly older than other associated phosphate grain types with Sr isotope ages similar to the microsphorites. They are interpreted to have formed as microsphorite on unconformity surfaces during the lowstand, were ripped up from these surfaces during the subsequent transgression, and reworked into the overlying sedi ent sequence. (3) Skeletal grains have quite variable Sr isotope ages, but in general they are younger than the other phosphate grain types. Most skeletal grains are interpreted to be contemporaneous with the peloids and foraminifera. However, their younger ages are interpreted to be in response to secondary geochemical processes associated with (a) recrystallization from carbonate hydroxyapatite to carbonate fluorapatite and (b) subsequent precipitation of apatite in the pore spaces characteristic of porous vertebrate skeletal material. Some skeletal material is older and probably has been reworked from the underlying unconformity surfaces.

BENTHIC FORAMINIFERA AND PALEOECOLOGY OF THE PLIOCENE YORKTOWN AND CHOWAN RIVER FORMATIONS, LEE CREEK MINE, NORTH CAROLINA, USA

Pliocene deposits assignable to the Yorktown Formation (Sunken Meadow, Rushmere, and Morgarts Beach Members) and Chowan River Formation (Edenhouse Member) yielded 129 species and subspecies of benthic foraminifera. Twenty-six taxa, selected on the basis of consistent occurrence (present in 50% or more of the samples) and relative abundance (3% or more of the benthic assemblage in at least one sample), account for 88% of the identifiable specimens. From among these 26, principal components analysis identified 14 species that account for nearly 90% of the faunal variation, and cluster analyses revealed distinct stratigraphic assemblages that in general conform to the lithologic subdivisions. The most dramatic faunal change corresponds to the boundary between the Sunken Meadow and Rushmere Members. Based upon the benthic foraminiferal faunas, the Sunken Meadow, Rushmere, and Morgarts Beach Members of the Yorktown Formation were deposited in middle to outer neritic, outer neritic, and middle neritic environments, respectively. The Edenhouse Member of the Chowan River Formation was deposited in an inner to middle neritic environment. Paleobathymetric interpretations based on benthic foraminifera are consistent with those based on other fossil groups. Paleotemperature implications of the benthic foraminiferal assemblages are not entirely consistent with previous studies. Whereas numerous studies have concluded that Pliocene temperatures at middle and high latitudes were significantly warmer than modern temperatures, the benthic foraminiferal faunas encountered in our study suggest paleotemperatures comparable to those of today. Evidence from benthic foraminifera at the Lee Creek Mine is not sufficient basis to question studies indicating warmer mid-latitude Pliocene seas. Rather, it is likely that localized conditions have overprinted the larger-scale climatic signal.