Advection Of Seawater Research Articles

Impact of mesoscale eddies on the source funnel of sediment trap measurements in the South China Sea

The mooring tethered time-series sediment traps (TS-traps) collect sinking particles in the ocean, enabling the estimation of biological pump (BP) efficiency based on the assumption of vertical settling. However, the advection of sea water can disperse particles over long distance during the sinking process, introducing uncertainties into the estimates of BP efficiency. In the South China Sea, mesoscale eddies generate significant lateral transport above the mesopelagic zone, which has a large impact on the collecting area and source funnel of TS-traps. We studied the role of eddies in the lateral transport of sinking particles using a Lagrangian Particle Tracking Model (LPTM) which was forced by three-dimensional currents of eddy composites derived from the global Hybrid Coordinate Ocean Model (HYCOM). The eddy reanalysis using the HYCOM outputs was validated using occurrence, genesis and dissipation statistics of eddies. Backward particle tracking was performed at three stations located in the northern, central and southwestern SCS, respectively. The results indicate that the vertical sinking assumption is invalid when the particle sinking velocity is relatively low. Furthermore, the fraction of surface sources from the edge of the eddies depends on both the sinking velocity and seasonal variability of the background current. The eddy current controls the movement of particles within the eddy, and many of the particles with a sinking velocity <80 m d−1 can be transported from the edge to the eddy center. The source funnel within the mesoscale eddy is tilted vertically, leading to an asymmetrical distribution of the particle source around the trap location. Finally, for slow-sinking particles, trap-collected particles could originate from the subsurface located tens to hundreds of kilometers away from the trap location. Overall, our results highlight the role of mesoscale eddies in interpreting trap measurements.

An abrupt Middle-Miocene increase in fluid flow into the Leeward Margin Great Bahama Bank, constraints from δ44Ca and Δ47 values

The advection of seawater into the sediments deposited on the margins of Great Bahama Bank has been demonstrated to play in important role in ventilating the uppermost sediments, as well as supplying elements for diagenetic reactions deeper within the platform. Here, we implement a numerical model to calculate the rate of fluid advection at ODP Site 1003, using calcium isotopes as a tracer for fluid advection. A key parameter for this model, the rate of recrystallization of sediments, was constrained using the clumped isotope proxy. The model was tuned to existing datasets of clumped and carbonate calcium isotope measurements for this site. Results of this modeling effort indicate that, prior to ∼15 Ma, fluids were advected into the platform at a rate substantially lower than the present day. The rate of advection abruptly increased ∼13 Ma to a value greater than the present day. The increase in advective flux during the mid-Miocene coincides with a major reorganization of the platform from a ramp-like geometry to a steeper carbonate platform, indicating a change in the relative contributions of different mechanisms governing fluid flow on the platform margin and interior. This paper aims to explore the utility of using clumped and calcium isotopes to quantitatively reconstruct past fluid advection rates using this novel technique.

Iodine-129 concentration in seawater near Fukushima before and after the accident at the Fukushima Daiichi Nuclear Power Plant

Abstract. Anthropogenic radionuclides were released into the environment in large quantities by the Fukushima Daiichi Nuclear Power Plant (1FNPP) accident. To evaluate accident-derived 129I, the 129I concentrations in seawater before and after the accident were compared. Before the accident (2008–2009), the 129I concentrations in the western margin of the North Pacific between 32° N and 44° N showed a latitudinal gradient that was expressed as a linear function of latitude. The highest and average 129I concentrations after the accident were 73 times and approximately 8 times, respectively, higher than those before the accident in this study area. Considering the distribution of 129I in surface seawater, the accident-derived 129I in the southern and northern stations of the 1FNPP was predominantly supplied by seawater advection and atmospheric deposition (including microbial volatilization), respectively. As of October 2011, depth profiles of 129I revealed that 129I originating from the 1FNPP existed mainly in the upper 100 m depth. From the depth profiles, the cumulative inventories of accident-derived 129I were estimated to be (1.6–9.6) × 1012 atoms m−2 in this study area. On the basis of the 129I data in the seawater near Fukushima, the effective dose of 129I from seafood ingestion was much smaller than the annual dose limit.

The effect of river runoff and phytoplankton production on the seasonal variation of the chemical composition of coastal waters of the Amursky Bay, Sea of Japan

The factors that control the seasonal variability of the concentration of suspended matter, nutrients, dissolved forms of metals and phytoplankton biomass in coastal waters of the eastern part of the Amurskii Bay were determined. It was shown that the runoff of the Razdolnaya River determined the seasonal variability in suspended matter content, values of the chemical oxygen demand (COD), silicates, Fe, Mn, Cu, and Ni significantly affected the concentration of nitrates. The advection of sea water is the main source of phosphates and dissolved forms of Zn and Cd. In the autumn and winter seasons, the gradual decrease in the concentration of silicates and phosphates was related to phytoplankton activity. In the spring and summer seasons, the growth of phytoplankton determined the variability of the phosphate and nitrate contents and compensated for the effect of the Razdolnaya River runoff. It is concluded that the phytoplankton biomass, rather than variations in riverine flow, caused the long-term variability of the nitrate and phosphate concentrations in the coastal waters of the Amurskii Bay.

Distribution of Pu isotopes in marine sediments in the Pacific 30 km off Fukushima after the Fukushima Daiichi nuclear power plant accident

Copyright © 2012 by The Geochemical Society of Japan. in seawater in the immediate vicinity of the Fukushima DNPP (TEPCO, 2011). From the analysis of 131I/137Cs activity ratios, Tsumune et al. (2012) found that the direct release of radioactive liquid wastes into the ocean started on March 26, 2011, and the direct release from the plant site contributed more to the measured 137Cs concentration than atmospheric deposition did. Using a regional ocean model, they further estimated that from March 26 to the end of May, the total amount of 137Cs directly released was 3.5 ± 0.7 PBq. In May–June, 2011, accident-released 134Cs and 137Cs were already being observed in coastal seawater along the Northern Sanriku coast and Tsugaru Strait, 250–450 km north of the Fukushima DNPP (Inoue et al., 2012). Honda et al. (2012) studied the distribution of 134Cs and 137Cs in surface seawater in the western North Pacific one month after the accident and found that accident-released radionuclides were distributed widely in the western North Pacific. The 137Cs observed there was transported not only by diffusion and advection of seawater but also via the atmosphere as an aerosol after the accident. Honda et al. (2012) estimated the deposition of 137Cs into the sea surface via the atmosphere was 0.18 PBq, which is smaller than the deposition of ca. 5 PBq estimated by Kawamura et al. (2011) using a numerical simulation. Distribution of Pu isotopes in marine sediments in the Pacific 30 km off Fukushima after the Fukushima Daiichi nuclear power plant accident

A note on boundary conditions for salt and freshwater balances

The derivation of surface boundary conditions for salt and freshwater budgets, in the presence of an imbalance of evaporation and precipitation, is revisited in order to: (i) clarify the physical concepts involved and (ii) derive consistent approximations. Proper surface boundary conditions can be obtained by assuming that the air–sea interface is a material surface for the salt continuum. This requires all the mass exchange at the air–sea interface to be fulfilled by the freshwater continuum. The need of diffusive fluxes is crucial for this assumption to hold. If the air–sea interface varies in time (either by seawater advection, an imbalance of evaporation minus precipitation, or a combination of both) there must be a corresponding salt flux, in order for the transport across such moving surface to be nil. Here it is essential to distinguish the flux vector of a property from its transport across a (possibly moving) surface. Only in a motionless ocean surface approximation, which includes the classical rigid lid approximation, the advective and diffusive salt fluxes perpendicular to the surface cancel out exactly.

Monthly stable isotope records in an Australian coral and their correspondence with environmental variables

Monthly skeletal stable isotope measurements of a Porites australiensis coral from Abraham Reef, Australia, spanning the period from 1981 to 1991, are compared with monthly averaged SST (sea surface temperature) from satellite collected AVHRR (Advanced Very High Resolution Radiometer) and monthly downward surface short‐wave insolation (SSI) measurements. We found that the δ18O values varied as a function of the change in seawater temperature. There were anomalously high δ18O values in coral that grew during the summer season of the three El Niño events 1982–83, 1987, and 1991. Advection of seawater with higher salinity (and higher δ18O values) to our site or discrepancies between ocean skin temperature and mixed layer SST are likely reasons for the reduced resolution of δ18O‐based SST. Increased coral δ13C values occurred simultaneously with increases in SSI. During most El Niño events the maximum and minimum δ13C values were lower than during normal years.

Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most

210Pb activities in and fluxes to sediments of the Washington continental slope and shelf

Surface sediments of the Washington coast have 210Pb activites which average 104 ± 48dpm/g for submarine canyon and slope regimes and 18 ± 12dpm/g for the continental shelf regime. 210Pb sedimentary fluxes are also higher in canyons, averaging 18 ± 13dpm/cm 2 per yr, compared to 5.2 ± 3.1 dpm/cm 2 per yr for slope and 4.8 ± 1.8dpm/cm 2 per year for shelf regions. These 210Pb activities and fluxes are 2–7 times greater than those reported for other coastal regions. Inputs from the atmosphere and the Columbia River are not sufficient to supply the 210Pb, but advection of seawater containing dissolved 210Pb produced in situ from 226Ra provides an input several times larger than the sedimentary fluxes. The sedimentary 210Pb flux is limited by scavenging reactions rather than by supply of dissolved 210Pb. Calculations of maximum biological uptake and fluxes of 210Pb and ‘selective’ chemical leaching experiments all show that the primary scavenging processes are due to hydrous Mn and Fe oxides rather than biological phases. The pattern of higher 210Pb depositional fluxes in canyons than in nearby open slope areas of comparable water depth is most reasonably explained by enhanced scavenging of dissolved 210Pb near the sea floor, rather than by processes operating throughout the water column. Relatively rapid removal of dissolved 210Pb from the near bottom nepheloid layer to slope and canyon sediments is shown by its mean residence time of less than two years in this layer.