David J. Mallinson

Origin and age of phosphorite from the south-central Florida Platform: Relation of phosphogenesis to sea-level fluctuations and δ13C excursions

Abstract Francolite (carbonate fluorapatite) from predominantly peloidal phosphorite grains in the Miocene Hawthorn Group from the Babcock Deep core (southwestern Florida) is highly substituted with an average composition of (Ca4.61Na0.22Mg0.14K0.05Sr0.02) (PO4)2.23(CO3)0.62(SO4)0.12(F)1.04. δ13C values of −1.0–−7.8 %. PDB of the francolite CO3 indicate a substantial contribution of C from an organic source. The sulfur isotopic composition of francolite SO4 is depleted to slightly enriched relative to Miocene seawater and suggests that phosphogenesis occurred during early burial diagenesis in organicrich sediments above or in the zone of sulfate reduction. The uniformly high Sr, Na, Mg, SO4, and CO3 contents of the francolite, the δ18O values of the francolite CO3 (−0.2–−4.2%. PDB), and the inclusion of pyrite and organic matter in phosphorite grains suggest that the francolite has had minimal chemical alteration since its formation. Phosphorite from the Babcock Deep core has Sr( 87 86 ) ratios that range from 0.708148-0.708880 and Sr-derived ages that range from late Oligocene (25.6 ± 0.7 Ma) to early late Miocene (9.2 ± 1.4 Ma). The Sr-derived ages of relatively fragile calcareous benthic foraminifers are consistently younger than associated hard, tightly cemented phosphorite and support textural evidence of extensive reworking. The age of the host sediment ranges from early Miocene to Pliocene and its depositional history is related to sea-level fluctuations. Approximately three-quarters of the total phosphorite in the Babcock Deep core formed during or within 2 my before the major early to mid-Miocene positive δ13Cshift as recorded at DSDP Site 588 (southwest Pacific). One-third of the total phosphorite formed just prior to the carbon shift in the short interval between 21.9 and 20.4 Ma. These results indicate that reworked phosphorite deposits may provide a proxy of organic carbon cycling on continental shelves that correspond to sealevel fluctuations and to variations in the δ13Crecord.

Development of small carbonate banks on the south Florida platform margin: response to sea level and climate change

Abstract Geophysical and coring data from the Dry Tortugas, Tortugas Bank, and Riley’s Hump on the southwest Florida margin reveal the stratigraphic framework and growth history of these carbonate banks. The Holocene reefs of the Dry Tortugas and Tortugas Bank are approximately 14 and 10 m thick, respectively, and are situated upon Pleistocene reefal edifices. Tortugas Bank consists of the oldest Holocene corals in the Florida Keys with earliest coral recruitment occurring at ∼9.6 cal ka. Growth curves for the Tortugas Bank reveal slow growth (

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS 14C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.

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.

Linking phosphogenic episodes on the southeast U.S. margin to marine δ13C and δ18O records

The relative abundance of phosphorite as a function of age was estimated for the southeastern United States using 87Sr/86Sr derived ages. The Sr-derived ages define seven phosphogenic episodes on the northeast Florida platform. Phosphorus burial rate estimates for the southeastern United States margin range from 2 to 12 × 109 mol ṁ P ṁ yr−1, and phosphorus accumulation rates range from 1 to 3 µmol ṁ cm−2 ṁ yr−1, similar to modern phosphogenic margins. The estimated amount of organic carbon burial ranges from 1 to 12 × 1016 mol C; sufficient to shift the ocean δ13C by 0.1‰ to 0.6‰. On long time scales (>1 m.y.) phosphogenesis in the southeastern United States corresponds to increased organic carbon burial (positive δ13C shifts) and rising sea level (negative δ18O shifts). On shorter time scales (<1 m.y.), the correlation of phosphogenesis to positive δ13C and δ18O shifts suggests a different relation between phosphogenesis, organic carbon burial, eustasy, and climate transitions during the Miocene.

Strontium isotopes and Miocene sequence stratigraphy across the northeast Florida Platform

ABSTRACT A preliminary assessment of Miocene sea-level fluctuations is presented based on the integration of Sr-derived ages and sequence-stratigraphic concepts applied to subsamples of 12 cores that transect the Hawthorn Group on the northeast Florida Platform. The Hawthorn Group section includes phosphorite nodules, peloids, and crusts as well as dolosilt and dolostone nodules and beds formed during early burial diagenesis of organic-rich sediments deposited during periods of intensified and persistent upwelling associated with rising and maximum sea level. The 87Sr/86Sr composition of phosphorite and dolomite is used to determine the age of in-place phosphorite crusts and dolostone beds (condensed sections) and reworked phosphorite and dolostone sand and gravel (unconformities and transgressive surfaces). Regional correlation of unconformities and condensed sections provides the basis for a sequence-stratigraphic framework from which the age and relative amplitude of sea-level fluctuations are constructed. Seven depositional sequences are identified from the data. Depositional sequences correspond to local sea-level fluctuations with maximum water depths at approximately 25-24, 21-20, 19-18, 17-15, 14-12, 11-9, and 8-6 Ma. Greatest water depth apparently occurred at 17-15 Ma.

Paleoclimate implications of high latitude precession-scale mineralogic fluctuations during early Oligocene Antarctic glaciation: the Great Australian Bight record

Sediments from ODP Site 1128 in the Great Australian Bight record isotopic and mineralogic variations corresponding to orbital parameters and regional climate change during the early Oligocene climate transition and Oi1 glacial event. Bulk carbonate stable isotope analyses reveal prominent positive oxygen and carbon isotope shifts related to the inferred major increase in glaciation at approximately 33.6 to 33.48 Ma. The oxygen isotope excursion corresponds to a prolonged period of low eccentricity, suggesting ice-sheet growth during low seasonality conditions. The clay mineralogy is dominated by smectite throughout. The exclusive occurrence of highly crystalline smectite from 33.6 to 33.5 Ma suggests the occurrence of explosive volcanism that correlates with the positive oxygen isotope shift. The dominance of mixed-layer smectite from 33.5 to 33.4 Ma and an increase in illite following 33.4 Ma indicates a transition from cool, wet conditions to cool, dry conditions over Australia during the Oi1 glaciation. Clay mineralogy and carbonate percentages reveal precession-scale oscillations during the Oi1 event. Kaolinite varies inversely with smectite and percent carbonate. Variations in precipitation and runoff, and wind velocities during southern hemisphere summer perihelion and high eccentricity intervals may account for the precession-scale oscillations. D 2003 Elsevier B.V. All rights reserved.

Geochemical consequences of increased Late Cenozoic weathering rates and the global CO2 balance since 100 Ma

Large imbalances in the relative net CO2 flux over the last 100 m.y. are obtained from independently derived estimates of CO2 uptake by weathering and organic carbon burial and of CO2 outgassing assumed proportional to the mean mid-ocean ridge (MOR) spreading and mantle plume production (MPP) rates. Increasing the river flux of Ca, Mg, P, and C by 50% since 30 Ma to parallel the proposed increase in the Sr river flux needed to explain the Sr isotopic evolution of seawater [Richter et al., 1992] has a large impact on the global carbon cycle by reducing the excess net CO2 imbalance in the late Cenozoic. CO2 uptake is calculated by assuming that 19% of the Ca river flux and 60% of the Mg river flux are derived from weathering silicates and that Mg is removed in the oceans by 1:1 molar exchange reactions with calcsilicates. An increase in the Mg river flux combined with an overall decrease in the MOR spreading rate predicts as much as a factor of 2 increase in the seawater Mg concentration since 100 Ma. The net organic carbon burial flux (burial minus weathering) largely reflects changes in the bulk carbonate δ13C record and, to a lesser extent, variations in the net carbonate burial flux and mean δ13C value of the carbonate weathering flux. Organic carbon burial efficiency is markedly less than that of P from the middle middle Miocene to the present because of early diagenetic transfer of organic P to francolite and oxidation of organic carbon during reworking of margin sediments by sea level fluctuations. The large CO2 imbalance of the Late Cretaceous requires significantly less carbonate subduction or greater weathering rates. A CO2 balance can be achieved in the Miocene by decreasing the proposed increase in river fluxes by up to half or by variably increasing the amount of carbonate subducted. Variations in shallow water carbonate burial, and hence the global carbonate record, are currently too poorly known to differentiate between the above possibilities. The decrease in the excess CO2 flux generally parallels the trend to cooler climate inferred from the δ18O record, but periods of large CO2 imbalance generally precede δ18O shifts by several million years.

Origin of Dolomite in the Phosphatic Miocene Hawthorn Group of Florida

ABSTRACT In addition to large amounts of phosphorite, the Miocene Hawthorn Group of Florida contains abundant dolomite. Dolomite is present as disseminated silt-size rhombs, as friable dolosilt beds, and as pore-filling cement in dolostone beds and clasts. The dolomite formed during early burial diagenesis both in the sulfate-reduction zone, overlapping and extending below sediment depths of phosphorite formation, and in adjacent, nonphosphatic, shallow-water lagoonal environments. Much of the dolomite is closely associated with the fibrous, Mg-rich clay minerals palygorskite and sepiolite. The percent carbonate in the Hawthorn Group increases from north to south; the dominant carbonate mineral in north Florida is dolomite, whereas dolomite and calcite are both abundant in south Florida. The 13C values of the dolomite, from +1.82 to -6.21 PDB, suggest that metastable biogenic carbonate (aragonite and high-Mg calcite) and seawater were the predominant sources of carbonate. However, negative 13C values of dolomite from northeast Florida suggest that as much as 30-40% of the carbonate was derived from degradation of organic matter. Degradation of organic matter enhanced dolomitization by removing sulfate ion and increasing the carbonate alkalinity of the pore waters. The oxygen and strontium isotopic values along with moderate Na contents indicate a marine origin. Evaporation of seawater or mixing o seawater and meteoric water were apparently not major factors in dolomite formation. The 87Sr/86Sr ratios of the dolomite range from 0.708129 to 0.708820 and correspond to Sr-derived ages of 26 to 12 Ma. The similarity of associated dolomite and phosphorite Sr-derived ages (r2 = 0.79) combined with geochemical and textural evidence suggest that dolomite and phosphorite formed at approximately the same time during rising or maximum sea level and were reworked during marine regressions. Reworking of the sediment concentrates disseminated dolomite rhombs and phosphorite grains, oxidizes organic matter, and exhumes buried dolostone beds to boring and encrusting organisms on the seafloor. Therefore, the presence of dolomite, along with phosphorite, in reworked sequences can indicate deposition of organic-rich sediments from which most of th organic matter has since been removed.