M. Baskaran

Historical contamination of PAHs, PCBs, DDTs, and heavy metals in Mississippi River Delta, Galveston Bay and Tampa Bay sediment cores

Profiles of trace contaminant concentrations in sediment columns can be a natural archive from which pollutant inputs into coastal areas can be reconstructed. Reconstruction of historical inputs of anthropogenic chemicals is important for improving management strategies and evaluating the success of recent pollution controls measures. Here we report a reconstruction of historical contamination into three coastal sites along the US Gulf Coast: Mississippi River Delta, Galveston Bay and Tampa Bay. Within the watersheds of these areas are extensive agricultural lands as well as more than 50% of the chemical and refinery capacity of the USA. Despite this pollution potential, relatively low concentrations of trace metals and trace organic contaminants were found in one core from each of the three sites. Concentrations and fluxes of most trace metals found in surface sediments at these three sites, when normalized to Al, are typical for uncontaminated Gulf Coast sediments. Hydrophobic trace organic contaminants that are anthropogenic (polycyclic aromatic hydrocarbons, DDTs, and polychlorinated biphenyls) are found in sediments from all locations. The presence in surface sediments from the Mississippi River Delta of low level trace contaminants such as DDTs, which were banned in the early 1970s, indicate that they are still washed out from cultivated soils. It appears that the DDTs profile in that sediment core was produced by a combination of erosion processes of riverine and other sedimentary deposits during floods. Most of the pollutant profiles indicate that present-day conditions have improved from the more contaminated conditions in the 1950-1970s, before the advent of the Clean Water Act.

Isotopic evidence for the contemporary origin of high-molecular weight organic matter in oceanic environments

Abstract Previous work has suggested that apparent old 14C ages for oceanic DOC are the result of mixing of different organic carbon fractions. This report provides direct evidence for a contemporary 14C age of a high-molecular-weight (HMW) fraction of colloidal organic carbon (≥10 kD). Colloidal organic matter, COM10 (from 10 kDaltons (kD) to 0.2 μm), isolated from the upper water column of the Gulf of Mexico and the Middle Atlantic Bight (MAB) region, generally has a contemporary age (i.e., younger than a few decades), while COM1 (from 1 kD to 0.2 μm), is apparently old: 380–4500 y b P. Thus, BMW COM10 (3–5% of DOC) from the upper water column is derived from living particulate organic matter (POM) and cycles rapidly, while a significant fraction of low-molecular-weight (≤1 kD) DOM is likely more refractory, and cycles on much longer time scales. The presence of pigment biomarker compounds in COM1 from the upper water column points to selected phytoplankton species as one of the sources of COM. Terrestrial carbon as another source of COM is suggested from the inverse correlation between Δ14C and δ13C values, as well as the increasing δ13C values with increasing salinity. 234Th-derived turnover times of COM10 and COM1 from both the Gulf of Mexico and MAB are consistently short, 1–20 and 3–30 days, respectively. These short residence times support the hypothesis that 14C ages of colloidal fractions of DOC are the result of COM fractions being a mixture of several endmembers with fast and slow turnover rates.

Sediment chronology in San Francisco Bay, California, defined by 210Pb, 234Th, 137Cs, and 239,240Pu

Abstract Sediment chronologies based on radioisotope depth profiles were developed at two sites in the San Francisco Bay estuary to provide a framework for interpreting historical trends in organic compound and metal contaminant inputs. At Richardson Bay near the estuary mouth, sediments are highly mixed by biological and/or physical processes. Excess 234 Th penetration ranged from 2 to more than 10 cm at eight coring sites, yielding surface sediment mixing coefficients ranging from 12 to 170 cm 2 /year. At the site chosen for contaminant analyses, excess 210 Pb activity was essentially constant over the upper 25 cm of the core with an exponential decrease below to the supported activity between 70 and 90 cm. Both 137 Cs and 239,240 Pu penetrated to 57-cm depth and have broad subsurface maxima between 33 and 41 cm. The best fit of the excess 210 Pb profile to a steady state sediment accumulation and mixing model yielded an accumulation rate of 0.825 g/cm 2 /year (0.89 cm/year at sediment surface), surface mixing coefficient of 71 cm 2 /year, and 33-cm mixed zone with a half-Gaussian depth dependence parameter of 9 cm. Simulations of 137 Cs and 239,240 Pu profiles using these parameters successfully predicted the maximum depth of penetration and the depth of maximum 137 Cs and 239,240 Pu activity. Profiles of successive 1-year hypothetical contaminant pulses were generated using this parameter set to determine the age distribution of sediments at any depth horizon. Because of mixing, sediment particles with a wide range of deposition dates occur at each depth. A sediment chronology was derived from this age distribution to assign the minimum age of deposition and a date of maximum deposition to a depth horizon. The minimum age of sediments in a given horizon is used to estimate the date of first appearance of a contaminant from its maximum depth of penetration. The date of maximum deposition is used to estimate the peak year of input for a contaminant from the depth interval with the highest concentration of that contaminant. Because of the extensive mixing, sediment-bound constituents are rapidly diluted with older material after deposition. In addition, contaminants persist in the mixed zone for many years after deposition. More than 75 years are required to bury 90% of a deposited contaminant below the mixed zone. Reconstructing contaminant inputs is limited to changes occurring on a 20-year time scale. In contrast, mixing is much lower relative to accumulation at a site in San Pablo Bay. Instead, periods of rapid deposition and/or erosion occurred as indicated by frequent sand-silt laminae in the X-radiograph. 137 Cs , 239,240 Pu , and excess 210 Pb activity all penetrated to about 120 cm. The distinct maxima in the fallout radionuclides at 105–110 cm yielded overall linear sedimentation rates of 3.9 to 4.1 cm/year, which are comparable to a rate of 4.5±1.5 cm/year derived from the excess 210 Pb profile.

Distribution of dissolved and particulate230Th and232Th in seawater from the Gulf of Mexico and off Cape Hatteras as measured by SIMS

We present vertical and horizontal profiles of dissolved and particulate230Th and232Th concentrations in seawater from shelf and slope areas of the Gulf of Mexico and off Cape Hatteras in the Middle Atlantic Bight, as measured by secondary ion mass spectrometry (SIMS), and compare the partitioning of long-lived230Th between particles and seawater with that of short-lived234Th. n nConcentrations of dissolved and particulate230Th and232Th in surface seawater decreased from near-shore to off-shore in the Gulf of Mexico, demonstrating terrestrial input to the surface waters. Vertical profiles indicate that lateral transport, as well as surface inputs (e.g., fluvial and aeolian) and bottom sediment resuspension, could be an important source of230Th and232Th in ocean margin regions. Sea water samples collected off Cape Hatteras contained significantly higher dissolved and particulate230Th and232Th concentrations than those found in the open Gulf of Mexico. Likely, the distributions of230Th and232Th in seawater off Cape Hatteras are, to a large extent, controlled by terrestrial particles. In contrast, in-situ production (for230Th) and scavenging are the major processes regulating the concentrations of dissolved230Th and232Th in the open Gulf of Mexico. The observed negative correlation between log Kd (particle-solution distribution coefficient) and log Cp (concentration of suspended particulate matter) for234Th and230Thxs in both study areas, demonstrated a “particle-concentration effect” down to very low Cp concentrations (∼ 20 μg/l). Based on the230Th inventory, in-situ production, and the small estimated surface input, the overall residence time of230Th in the water column at a slope station in the Gulf of Mexico is calculated to be about 5 yr.

An isotopic biogeochemical assessment of shifts in organic matter input to Holocene sediments from Mud Lake, Florida

Abstract A molecular and isotopic study of sediment cores from a sinkhole lake, Mud Lake, Florida, USA, was performed in order to relate documented changes in the regional terrestrial vegetation and water table over the last ∼5500 years to molecular and isotopic proxies for biological sources of organic matter to the lake sediments. Temporal shifts in the source of organic matter to the sediment, as determined by the stable carbon-isotope composition of bulk organic matter and biomarkers, correspond with previously defined regional scale transitions in forest ecosystems ( Quercus to Pinus at ∼5500 14 C yr BP and Taxodium expansion at 2500 14 C yr BP) and coincident increases in the Floridan water table. The δ 13 C values for total sedimentary organic carbon showed a shift from terrestrial and aquatic macrophyte sources (−27.8‰, in sediments dated at ∼5400 14 C yr BP) within the basal peat, to a cyanobacterial-dominated sapropel (−18.1‰) at the surface. A comparison of the δ 13 C values of bulk sediment and biomarkers representative of cyanobacterial and algal input (e.g. 7- and 8-methylheptadecane and the n -alkane C 17 ) indicates that the present shallow lake was fully developed by ∼2400 14 C yr BP. Differences among the δ 13 C values of specific biomarkers derived from vascular plants (C 29 and C 31 n -alkanes and CuO lignin oxidation products) and from cyanobacteria and algae are nearly equivalent in magnitude to the shift recorded in δ 13 C of TOC, indicating their effective use as source proxies in this system.

234Th:238U disequilibria in the Gulf of Mexico: the importance of organic matter and particle concentration

We have measured the concentrations of dissolved and particulate234Th, suspended particulate matter, dissolved organic carbon, nutrients and a suite of pigments along a transect in the shelf and slope waters of the Gulf of Mexico. The deep water station samples were collected in the presence (summer 1992) and absence (spring 1992) of a warm-core ring. In the summer, the station was at the periphery of a warm-core ring containing more oligotrophic, Caribbean water, while in the spring, only Gulf water was present. The234Th deficiency, which extends throughout a 1500 m water column in spring, indicates active lateral exchange of dissolved and particulate species. This study provides insight into the role of particles for234Th scavenging and differences in the scavenging intensities in the presence and absence of a warm-core ring. In addition, the possibility that the major carrier phase of thorium cycling could be dissolved organic carbon is suggested from the correlation between234Th (dissolved and total) and dissolved organic carbon concentrations. The average particle residence time for the whole water column in spring, seven days, was significantly shorter than that for summer, 21 days. This longer residence time in the summer is attributed to the presence of a warm-core ring which resulted in lower particle, DOC, and nutrient concentrations. Values of scavenging and particle residence times and234Th fluxes, and the significance of the correlations of these values with concentrations of particle mass and various pigments are discussed.

Historical trace element distribution in sediments from the Mississippi River delta

Five sediment cores were collected on the shelf of the inner Mississippi Bight in June 2003 for a suite of radionuclides to establish geochronologies and trace elements to examine patterns of contaminant deposition and accumulation. Core sites were chosen to reflect a matrix of variable water depths, proximity to the Mississippi River mouth as the primary source for terrigenous particles, and extent and duration of summertime water column hypoxia. The vertical distribution of239,240Pu and210Pbxs(=210Pbtotal−226Ra) provided reliable geochronological age constraints to develop models for mass accumulation rates and historic trace element inputs and variations. Mass accumulation rates ranged from 0.27 to 0.87 g cm−2yr−1 and were internally consistent using either210Pbxs or239,240Pu. Measured inventories of137Cs,239,240Pu, and210Pbxs were compared to atmospheric deposition rates to quantify potential sediment focusing or winnowing. Observed variability in calculated mass accumulation rates may be attributed foremost to site-specific proximity to the river mouth (i.e., sediment source), variability in water depth, and enhanced sediment focusing at the Mississippi River canyon site. Trace element concentrations were first normalized to Al, and then Al-normalized enrichment factors (ANEF) were calculated based on preanthropogenic and crustal trace element abundances. These ANEFs were typically >1 for V and Ba, while for most other elements studied, either no enrichment or depletion was observed. The enrichment of Ba may be related, in part, to the seasonal occurrence of oxygen-depleted subsurface waters off the Mississippi River delta, as well as being an ubiquitous byproduct of the petroleum industry.

Particulate and dissolved 210Pb activities in the shelf and slope regions of the Gulf of Mexico waters

Abstract Concentration profiles of dissolved and particulate 210 Pb were measured on horizontal transects of the continental shelf and slope regions of the Gulf of Mexico. Vertical profiles in the slope station in the presence and absence of warm-core ring enable us to evaluate the differences in the scavenging intensity of 210 Pb. A comparison of the rate of production of 210 Pb in the overlying water column to that of its mean annual flux calculated from the sediment inventory allows us to get a better insight in to the lateral transport and deposition of 210 Pb to the slope sediments over a Our results indicate that there is no general increase of dissolved or particulate 210 Pb concentration with depth. The scavenging residence time of 210 Pb for the entire 1500-m water column varied between 0.06 and 4.7 years. There was a strong correlation between the inventory of total (dissolved+particulate) 210 Pb and depth of the water column. The ratio of measured sediment inventory (F) of 210 Pb to its production (P) in the overlying water column varied between 0.69 and 1.63, suggesting that, at time, lateral inputs from boundary scavenging in a few sites are dominant, while in other places, vertical scavenging is the primary mechanism of transport for 210 Pb.

Heavy metals in Chukchi Sea sediments as compared to selected circum-arctic shelves

Abstract Surficial sediments from 31 stations on the northeastern Chukchi Sea, Alaskan Arctic were analysed by AAS and ICP-AES for Si, Al, Fe, Mn, Cu, Cr, Co, Zn, Ni and V. The metal concentrations are related to the sediment granulometry and show no correlations with organic carbon. Presumably all the heavy metals, except Cr and Co, are either adsorbed on clays and/or coprecipitated with ferrimanganic hydroxide. Cobalt is partitioned in an unknown mineral phase within the sand fraction. Comparison of the heavy metal concentrations in muds of the Chukchi Sea with those of the high arctic shelves of Russia, East Greenland and the Beaufort Sea shows relatively lower levels of most metals in the Chukchi. Presumably these disparities are related to regional differences in sediment chemistries inherited from natural terrigenous sources rather than to any enhanced pollution in the non-Chukchi areas. It is suggested that the concentrations and accumulation rates of the heavy metals reported here can serve as reliable baselines to monitor metal pollution in the Chukchi Sea.

Organic carbon flow in the Ob, Yenisey Rivers and Kara Sea of the Arctic region.

Stable carbon isotope and elemental C/N ratios of the organic fraction of a set of samples along a transect in the Ob and Yenisey Rivers into the Kara Sea in the Arctic were measured. Previously, the concentrations of 239,240Pu and 137Cs in these same samples had been determined. The coupled measurements were carried out to assess possible connectivity between organic carbon flow into the Kara Sea and transport of radioactive nuclides in this marine environment. Organic carbon flow into the Kara Sea is influenced significantly by terrigenous sources carried by the Ob and Yenisey Rivers. The carbon isotope-organic carbon relationship provides evidence that a rich source of terrigenous carbon exists in the riverine system. A weak, but significant relationship between stable carbon isotope ratio and 137Cs suggests that most of the 137Cs is derived from riverine particles, as compared to Pu which is also derived from in situ scavenging within the water column.