Robert C. Aller

Diagenetic processes near the sediment-water interface of Long Island Sound. I. Decomposition and nutrient element geochemistry (S,N,P)

Publisher Summary This chapter focuses on diagenetic processes near the sediment–water interface of Long Island Sound (LIS). Analysis of the solid phase in this study shows extensive and comparable production of sulfide fixed as FeS (acid-volatile sulfide) in all areas of LIS. Deep-water stations show extensive loss of solid-phase sulfide either prior to or after conversion of FeS to FeS2 (pyrite). All stations show similar seasonal variation in pore-water profiles near the sediment-water interface. These are periodic and repeatable from year to year. A two-dimensional transport-reaction model incorporating both radial transport into burrows and vertical diffusion is presented. This model is capable of predicting both the form and magnitude of pore-water profiles extraordinarily well at all stations. A one-dimensional model in which an effective transport coefficient is used to account for the influence of reworking and burrow construction on solute movement is less satisfactory in predicting the observed profiles. Equilibrium calculations indicate that HPO2−4 concentrations, in some cases, may be saturated with respect to various Ca, Mn, and Fe phosphates. The chapter demonstrates that the chemical composition of the bioturbated zone of marine sediments, like that of any natural body, represents a balance between transport and reaction processes taking place within and around it.

Effects of biological sediment mixing on the210Pb chronology and trace metal distribution in a Long Island Sound sediment core

An experiment was designed to assess the relative importance of sediment accumulation and bioturbation in determining the vertical distribution of nuclides in estuarine sediments. A diver-collected core, 120 cm long, was raised from central Long Island Sound and analyzed down its length for:210Pb and226Ra;239, 240Pu; and Mn, Zn, Cu, and Pb. Sampling for chemical analysis was guided by X-radiography of the core. Excess210Pb (relative to226Ra) is roughly homogeneous in the top 2–4 cm of the core, then decreases quasi-exponentially to zero at (or above) 15 cm.239, 240Pu and excess Zn, Cu, and Pb, relative to background values at greater depths in the core, are distributed like excess210Pb in the top 10–15 cm. The absence of Mn enrichment at the top of the core, in contrast to other cores raised from this station, suggests that 1–3 cm of sediment was lost by erosion at the site of this core sometime prior to sampling. Below 15 cm excess210Pb and excess Zn, Cu, and Pb are found only in the bulk sample from 25 to 30 cm and in clearly identifiable burrow fillings dissected from 70 cm and 115 cm depth. Infilling of large burrows, excavated and then abandoned by crustaceans, is therefore a mechanism for transfer of surficial material to depth in these sediments. The bioturbation rate in the top several centimeters at this station has been determined previously using234Th (24-day half-life). The distribution of239, 240Pu can be used to estimate a bioturbation rate for the underlying layer (to ∼10 cm depth); this rate is found to be 1–3% of the maximum mixing rate for the top 2–3 cm. Using these two mixing rates in a composite-layer, mixing + sedimentation model, the distribution of excess210Pb in the top 15 cm was used to constrain the sediment accumulation rate, ω. While the apparent rate of sediment accumulation (assuming no mixing below 2–4 cm) is 0.11 cm/yr, the model requires ω < 0.05 cm/yr. Thus in an area of slow sediment accumulation, a low rate of bioturbation below the surficial zone of rapid mixing causes an increase of at least a factor of two in apparent accumulation rate.

234Th/238U disequilibrium in near-shore sediment: Particle reworking and diagenetic time scales

The distribution of 234Th(t1/2 = 24.1days) in excess of its parent238U in the upper layers of near-shore sediment makes possible the evaluation of short-term sediment reworking and diagenetic rates.234Th has a maximum residence time in Long Island Sound water of 1.4 days. Seasonal measurement of234Th/238U disequilibrium in sediment at a single station in central Long Island Sound demonstrates rapid particle reworking and high234Thxs (>1 dpm/g) in the upper 4 cm of sediment with slower, irregular reworking and low234Thxs to at least 12 cm. The rate of rapid particle reworking varies seasonally and is highest in the fall. The rapidly mixed zone is characterized by steep gradients in sediment chemistry implying fast reactions spanned by234Th decay time scales.238U is depleted in the upper mixed zone and shows addition in reducing sediment at depth.

Diagenetic Processes Near the Sediment-Water Interface of Long Island Sound. II. Fe and Mn

Publisher Summary This chapter analyzes diagenetic processes for Fe and Mn near the sediment–water interface of Long Island Sound (LIS). The results obtained in this study show that pore-water profiles of Fe2+ and Mn2+ from three stations in LIS have general depth-dependent concentration distributions similar to those reported from other sedimentary basins: concentrations rise above seawater values to a maximum below the interface and then decrease again or remain constant deeper in the deposit. It is found that the production of Mn2+ in pore waters is directly related to the rate of reduction of Mn oxides during the decomposition of organic matter, both as a function of depth in the sediment as well as seasonally. Fe2+ is produced both by the reduction of Fe oxides and by abiogenic or biogenic oxidation of Fe sulfides. The temporal changes in both Mn2+ and Fe2+ profiles are repeatable from year to year. During the summer, pore-water Mn2+ in the top few centimeters reaches the highest concentration of the year. In the fall, Mn2+ concentrations are lowered in magnitude throughout the sediment column as a result of both decreased production.

Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments: Evidence from7Be,210Pb, and239,240Pu

Abstract Cosmogenic 7 Be(t 1/2 = 53.3days) has been used to estimate particle-mixing rates in the upper layers of lacustrine and near-shore marine sediments. Excess 210 Pb and/or 239,240 Pu have provided limits on rates of sediment accumulation in these environments and indices of the efficiency of the sediments as collectors of reactive nuclides over longer time scale. In sediment cores from Long Island Sound (marine) and Lake Whitney (fresh-water) 7 Be was measurable in the top 2–3 cm. Diffusion-analog particle-mixing coefficients calculated from these data are in the range of 10 −7 cm 2 /s. For Long Island Sound the coefficients are lower by factors of 3–6 than those estimated from the depth distributions of excess 234 Th at the same stations [14]. For Lake Whitney the calculated mixing coefficient is an upper limit because of the possibility of a sampling artifact. Measurements of total (wet + dry) atmospheric deposition of 7 Be in New Haven give an average flux of 0.07 dpm/cm 2 day during March-November, 1977; this is equivalent to a steady-state inventory of 5.4 dpm/cm 2 in a perfect collector. Sediment cores from Long Island Sound contain about half this 7 Be inventory, consistent with either a mean residence time for 7 Be in the water column of about one half-life or with post-depositional loss of 7 Be from Long Island Sound sediments. The Lake Whitney cores contain about 5 dpm/cm 2 , much nearer the atmospheric delivery. A higher inventory of 7 Be in fresh-water, as compared to marine, sediments could be due either to a shorter mean residence time for 7 Be in fresh water or to lateral transport processes in the lake or its catchment. High inventories of excess 210 Pb and 239,240 Pu in Lake Whitney sediments demonstrate the importance of lateral transport on longer time scales at least.

The Sources and Sinks of Nuclides in Long Island Sound

Publisher Summary This chapter provides an overview of the sources and sinks of nuclides in Long Island Sound (LIS). The LIS has the properties of a large protected settling basin. In such a system, many of the processes affecting the behavior of material injected into the coastal zone can be followed more directly than in some other systems more responsive to large-scale and seasonal variables. The Connecticut River is the most important river draining into LIS. This river is least important in the transport of metals to LIS. The distribution of trace metals in LIS sediments is discussed in the chapter. Trace-metal distributions in mussels and oysters are studied. The cause of the observed trace-metal distributions is analyzed, and the processes affecting the deposition and accumulation of trace metals in LIS are outlined. Processes that affect the vertical distribution of nuclides in the sediment pile are also described.

Particle reworking in sediments from the New York Bight apex: Evidence from 234Th/238U disequilibrium

Two diver-collected cores of mud-rich sediment from the New York Bight apex have been analyzed for 238 U and 232 Th decay series nuclides with emphasis on 234 Th/ 238 U disequilibrium. Excess 234 Th is present in both cores and shows exponential decrease in the top 4 cm. Biogenic reworking by a deposit-feeding community characterized by a Nucula proxima-Nephtys incisa assemblage apparently controls the form of the 234 Th profiles. Mixing coefficients of 0·3–0·6×10 −6 cm 2 s −1 are calculated. no decrease of 210 Pb with depth (0–11 cm) is found nor is there any vertical structure in profiles of 228 Th. Episodic deposition followed by periods of stability cause the observed homogeneity of the longer lived nuclides as well as laminated horizons in the sediment. 234 U/ 238 U isotope ratios reflect addition of either sea water or sewage source uranium to the collection area.