Larry K. Benninger

Pleistocene sea levels from raised coral reefs of haiti.

The elevations and ages of a sequence of three uplifted Pleistocene coral reefs on the Northwest Peninsula of Haiti have been determined. With the assignment of a sea level of +6 meters (relative to the present day) at 130,000 years before present and constant uplift of the reefs, the data indicate that sea level stood –10 and –13 meters at 108,000 and 81,000 years before present, respectively. These results are in substantial agreement with those reported for Barbados and New Guinea and support the hypothesis of constant uplift for each area. Sea level data from raised reefs indicate that the interglacial marine oxygen isotope oscillations during oxygen isotope stage 5 are a result of 30 percent ice volume effects and 70 percent temperature effects.

The coprecipitation of Pu and other radionuclides with CaCO3

The record of fallout plutonium concentrations in annual bands of corals is strikingly similar to the record of atmospheric deposition of 90Sr. This similarity implies that corals may incorporate Pu from seawater with a constant partition coefficient (constant discrimination). To investigate physicochemical aspects of Pu incorporation, the following have been coprecipitated with CaCO3 (calcite and aragonite): oxidized and reduced Pu; americium, thorium and uranium as analogs to Pu oxidation states (III, IV, VI), respectively; and 210Pb as a particle-reactive nuclide which may be incorporated by corals with constant discrimination. Americium, thorium and lead adsorb onto both calcite and aragonite, with more than 99% of the recovered activity found associated with the solids. Uranium exhibits a behavior consistent with lattice substitution. Partition coefficients for U in aragonite range from 1.8 to 9.8 and vary inversely with pH and/or rate of precipitation. The partition coefficient for U in calcite is less than 0.2 and may be as low as 0.046. Reduced Pu sorbs with 3 to 4% remaining in solution. Oxidized Pu may both sorb and coprecipitate. The coral record for Pb and U results primarily from biological, rather than physicochemical, effects; it is likely that the Pu coral record also reflects biological discrimination.

BENTHIC FLUXES AND POREWATER CONCENTRATION PROFILES OF DISSOLVED ORGANIC CARBON IN SEDIMENTS FROM THE NORTH CAROLINA CONTINENTAL SLOPE

Abstract Numerous studies of marine environments show that dissolved organic carbon (DOC) concentrations in sediments are typically tenfold higher than in the overlying water. Large concentration gradients near the sediment–water interface suggest that there may be a significant flux of organic carbon from sediments to the water column. Furthermore, accumulation of DOC in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and/or adsorption reactions. We measured DOC concentration profiles (for porewater collected by centrifugation and “sipping”) and benthic fluxes (with in situ and shipboard chambers) at two sites on the North Carolina continental slope to better understand the controls on porewater DOC concentrations and quantify sediment–water exchange rates. We also measured a suite of sediment properties (e.g., sediment accumulation and bioturbation rates, organic carbon content, and mineral surface area) that allow us to examine the relationship between porewater DOC concentrations and organic carbon preservation. Sediment depth-distributions of DOC from a downslope transect (300–1000 m water depth) follow a trend consistent with other porewater constituents (ΣCO2 and SO42−) and a tracer of modern, fine-grained sediment (fallout Pu), suggesting that DOC levels are regulated by organic matter remineralization. However, remineralization rates appear to be relatively uniform across the sediment transect. A simple diagenetic model illustrates that variations in DOC profiles at this site may be due to differences in the depth of the active remineralization zone, which in turn is largely controlled by the intensity of bioturbation. Comparison of porewater DOC concentrations, organic carbon burial efficiency, and organic matter sorption suggest that DOC levels are not a major factor in promoting organic matter preservation or loading on grain surfaces. The DOC benthic fluxes are difficult to detect, but suggest that only 2% of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment-water interface.

Seasonal variations in 7Be activity in the sediments of Cape Lookout Bight, North Carolina

Abstract Short-term sediment accumulation rates were determined using activity distributions of 7Be in the surficial sediments of a station (A-1) in Cape Lookout Bight, North Carolina (USA). This semienclosed coastal marine basin is characterized by high annual accumulation rates. Protected from high energy conditions by its geomorphology, the relatively quiescent waters of the bights interior cause it to act as an efficient settling trap for fine-grained sediment. Lack of bioturbation in the surface sediments allowed for variations in depth-integrated activity profiles of 7Be to be interpreted as short-term accumulation events. Beryllium-7 inventories exhibited seasonal cyclicity with inaxima occurring during spring (February through June). The inventory of 7Be ranged from 5.8 to 32.6 dpm cin−2 and was always well in excess of the atmospherically supported value ( x = 3.8 dpm cm −2 ; n = 15) . Accumulation rates calculated froin 7Be activity profiles indicate that the delivery of particulate matter to the sediments of Cape Lookout Bight is not constant throughout an annual cycle. The highest monthly accumulation rates appeared to be associated with north/northeast storm activity. Inputs were generally highest during late winter/early spring when storm frequency is greatest. Short-term accumulation rates derived from this study agree with 210Pb rates calculated for a ten-year period of accumulation. The annual sediment accumulation rates for each of the two years over which the study was conducted was found to be 4.8 ±0.8 g cm−2y−1 and 3.7 ± 1.2g cm−2y−1.

Sources of sediment to the Neuse River estuary, North Carolina

The Neuse River estuary is part of the large Albemarle/Pamlico estuarine/lagoonal system (APES) of North Carolina. Exchange between the APES and the open ocean is restricted to inlets in the Outer Banks barrier islands. Freshwater discharge of the Neuse River is low in relation to the size of its estuary, so that the estuary is normally brackish over most of the area which accumulates fine-grained sediments. Mud (silt + clay) accumulation rates are low (⩽ 6 mm year−1). From the mud distribution and the dynamics of suspended sediment in the estuary it has been inferred that the Neuse retains fines efficiently, with probable episodic (high discharge) losses seaward, to Pamlico Sound. As part of a study of particle transport, deposition, and accumulation in the Neuse estuary, we have collected radiochemical data (10 cores) and chemical data (seven cores) on sediment cores from the main channel of the estuary between New Bern, North Carolina and the estuary mouth. K/Al increases in the lower estuary, consistent with earlier reports of increasing illite in the clay fraction. Landward transport of sediment from Pamlico Sound is the most plausible explanation for the increasing K/Al (illite). A marine sediment source may, therefore, be important for the lower estuary. The distribution of sediment inventories (quantities per cm2 of sediment column) of particle-reactive radionuclides is consistent with the hypothesis of landward sediment transport. Inventories of excess 210Pb, 137Cs, and 239,240Pu are coherent through the estuary and show the effects of particle redistribution processes on regionally uniform inputs. To obtain excess (anthropogenic) Zn and Cu, we use metal/Al normalization to estimate natural backgrounds. Anthropogenic metal inputs are concentrated at the head of the estuary, and sediment inventories of excess Zn and Cu generally decrease seaward. Normalization of the excess Zn and excess Cu inventories to excess 210Pb inventories corrects the raw metal inventories for effects of lithology and sediment redistribution. The normalized excess Zn inventory decreases smoothly seaward, while the normalized excess Cu inventory shows an unexplained mid-estuary maximum. Low normalized inventories of anthropogenic metals at the estuary mouth indicate that little modern riverine sediment is stored there. Increasing K/Al (illite) and decreasing anthropogenic Zn and Cu in the lower estuary are thus both consistent with a predominantly marine sediment source. We conclude that landward transport of muds from Pamlico Sound has contributed significantly to the sediment balance of the lower Neuse estuary.

Mineral associations and nutritional quality of organic matter in shelf and upper slope sediments off Cape Hatteras, USA: a case of unusually high loadings

Abstract Relationships among organic carbon (OC), enzymatically hydrolyzable amino acid (EHAA) concentrations and mineral surface area (SFA) were assessed for sediments from the shelf and slope region near Cape Hatteras, USA. Grain size, measured here as mineral-specific surface area, explained 55% of the variance in organic matter concentrations. Organic loadings, as ratios of organic carbon to surface area (OC:SFA), decrease with water column and core depth. OC:SFA ratios in this region are comparable to those found in areas with anoxic water columns, and are the highest reported for shelf-slope sediments underlying oxygenated water columns. With increasing water-column depth, organic matter becomes progressively incorporated into low-density ( −3 ), organomineral aggregates, reflecting the increase in clay content in sediments with water-column depth. Organic coverage of mineral surfaces was determined by gas sorption methods; throughout the depth range minerals are essentially bare of organic coatings, in spite of high organic loadings. EHAA concentrations increase with water-column depth in a similar fashion as OC concentrations, and help to support intense heterotrophic communities at depth. Rapidly decreasing ratios of EHAA to total organic matter with water-column depth are consistent with previous inferences that largely refractory organic matter is exported from the shelf to the slope. Significant burial of EHAA downcore indicates protection of enzymatically hydrolyzable biopolymers with depth.

Fallout plutonium and natural radionuclides in annual bands of the coral Montastrea annularis, St. Croix, U.S. Virgin Islands

Abstract We have investigated the banded coral Montastrea annularis as a recorder of the history of fallout Pu in surface seawater. Thirty annual growth bands, representing growth during ‘coral years’ 1951–1980, were subsampled from M. annularis collected at St. Croix, U.S. Virgin Islands. ‘Coral years’ begin and end in late summer-early autumn and are designated for the calendar year in which they end. 239,240 Pu was finite in coral years 1954–1980, and the coral Pu record is very simply related to the fallout history of 90 Sr. Peaks in coral Pu in coral years 1959 and 1964 correspond to fallout peaks in 1959 and 1963, respectively. Peak broadening and time lags in the coral Pu record, as compared to the 90 Sr fallout record are consistent with retention of fallout Pu in surface seawater for about two years (characteristic removal time) during the period of major fallout, and possibly longer thereafter. The simplicity of the coral Pu record and its close correspondence with fallout history suggest that 239,240 Pu was incorporated into the coral skeleton with constant discrimination relative to Ca; the effects of speciation and oxidation state upon Pu incorporation are presently unknown. To aid the Pu interpretation Ca, Mg, Sr, Na and natural radionuclides ( 238 U, 228 Ra, 232 Th and 210 Pb) were also determined in the annual bands. In small samples (0.5 g) Ca, Mg and Na show correlated variations which could be due to seasonal variability in uptake. The 238 U and 228 Ra records were generally consistent with uptake, at constant discrimination, from surface-water reservoirs of nearly constant concentration, although one sample showed probable diagenetic addition of U. 232 Th was not detected with certainty; this implies that terrigenous particles were not consistently entrapped within the coral skeleton. Interpretation of 210 Pb was difficult because 226 Ra was not measured. Montastrea annularis preserves a record of fallout Pu. To make this record useful it must be considered in the broadest possible geochemical context.

Organic carbon inventories in natural and restored Ecuadorian mangrove forests

Mangroves can capture and store organic carbon and their protection and therefore their restoration is a component of climate change mitigation. However, there are few empirical measurements of long-term carbon storage in mangroves or of how storage varies across environmental gradients. The context dependency of this process combined with geographically limited field sampling has made it difficult to generalize regional and global rates of mangrove carbon sequestration. This has in turn hampered the inclusion of sequestration by mangroves in carbon cycle models and in carbon offset markets. The purpose of this study was to estimate the relative carbon capture and storage potential in natural and restored mangrove forests. We measured depth profiles of soil organic carbon content in 72 cores collected from six sites (three natural, two restored, and one afforested) surrounding Muisne, Ecuador. Samples up to 1 m deep were analyzed for organic matter content using loss-on-ignition and values were converted to organic carbon content using an accepted ratio of 1.72 (g/g). Results suggest that average soil carbon storage is 0.055 ± 0.002 g cm−3 (11.3 ± 0.8% carbon content by dry mass, mean ± 1 SE) up to 1 m deep in natural sites, and 0.058 ± 0.002 g cm−3 (8.0 ± 0.3%) in restored sites. These estimates are concordant with published global averages. Evidence of equivalent carbon stocks in restored and afforested mangrove patches emphasizes the carbon sink potential for reestablished mangrove systems. We found no relationship between sediment carbon storage and aboveground biomass, forest structure, or within-patch location. Our results demonstrate the long-term carbon storage potential of natural mangroves, high effectiveness of mangrove restoration and afforestation, a lack of predictability in carbon storage strictly based on aboveground parameters, and the need to establish standardized protocol for quantifying mangrove sediment carbon stocks.

How war, drought, and dam management impact water supply in the Tigris and Euphrates Rivers

The fast-paced conflicts in the Middle East can disrupt management and supply of water, particularly on dams and barrages along the Tigris and Euphrates rivers that have experienced threats or changes in sovereignty. Water supply is also under pressure from upstream water management, drought, and structural decline. In this research, we used a satellite-based algorithm, the normalized difference water index (NDWI), to monitor changes in the extent of surface reservoirs (1985-present). We compared the timeline of reservoir fluctuations with the timeline of events related to conflicts, droughts, and dam management. Our results show that the most sudden changes in water supply occurred during events related to conflict, but conflict was not often a cause of the greatest absolute changes to reservoir area. Though not as precise as on-the-ground information, satellite data can give insights to water supply when conflict has disrupted the flow of information or restricted on-the-ground data collection.