Michiel M Rutgers van der Loeff

An assessment of the use of sediment traps for estimating upper ocean particle fluxes

This review provides an assessment of sediment trap accuracy issues by gathering data to address trap hydrodynamics, the problem of zooplankton “swimmers,” and the solubilization of material after collection. For each topic, the problem is identified, its magnitude and causes reviewed using selected examples, and an update on methods to correct for the potential bias or minimize the problem using new technologies is presented. To minimize hydrodynamic biases due to flow over the trap mouth, the use of neutrally buoyant sediment traps is encouraged. The influence of swimmers is best minimized using traps that limit zooplankton access to the sample collection chamber. New data on the impact of different swimmer removal protocols at the US time-series sites HOT and BATS are compared and shown to be important. Recent data on solubilization are compiled and assessed suggesting selective losses from sinking particles to the trap supernatant after collection, which may alter both fluxes and ratios of elements in long term and typically deeper trap deployments. Different methods are needed to assess shallow and short- term trap solubilization effects, but thus far new incubation experiments suggest these impacts to be small for most elements. A discussion of trap calibration methods reviews independent assessments of flux, including elemental budgets, particle abundance and flux modeling, and emphasizes the utility of U-Th radionuclide calibration methods.

230Th normalization: An essential tool for interpreting sedimentary fluxes during the late Quaternary

There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230Th from the decay of dissolved 234U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.

Comparison of summer and winter inorganic carbon, oxygen and nutrient concentrations in Antarctic sea ice brine

Abstract During summer (January 1991) and winter (April 1992) cruises to the southern Weddell Sea (Antarctica), brine samples were collected from first year sea ice and analysed for salinity, temperature, dissolved oxygen and major nutrient concentrations. Additionally, the carbonate system was determined from measurements of pH and total alkalinity. During winter, brine chemical composition was largely determined by seawater concentration in the course of freezing. Brine temperatures ranged from −1.9 to −6.7 °C. Precipitation of calcium carbonate was not observed at the corresponding salinity range of 34 to 108. Removal of carbon from the total inorganic carbon pool (up to 500 μmol C t kg −1 ) was related to reduced nutrient concentrations, indicating the presence of photosynthetically active ice algal assemblages in the winter sea ice. However, nutrient and inorganic carbon concentrations did generally not reach growth limiting levels for phytoplankton. The combined effect of photosynthesis and physical concentration resulted in O 2 concentrations of up to 650 μmol kg −1 . During summer, brine salinities ranged from 21 to 41 with most values > 28, showing that the net effect of freezing and melting on brine chemical composition was generally slight. Opposite to the winter situation, brine chemical composition was strongly influenced by biological activity. Photosynthetic carbon assimilation resulted in a C t depletion of up to 1200 μmol kg −1 , which was associated with CO 2 (aq) exhaustion and O 2 concentrations as high as 933 μmol kg −1 . The concurrent depletion of major nutrients generally corresponded to uptake ratios predicted from phytoplankton biochemical composition. Primary productivity in summer sea ice is apparently sustained until inorganic resources are fully exhausted, resulting in brine chemical compositions that differ profoundly from those of surface waters. This may have important implications for pathways of ice algal carbon acquisition, carbon isotope fractionation as well as for species distribution in the open water phytoplankton.

Similar glacial and interglacial export bioproductivity in the Atlantic Sector of the Southern Ocean: Multiproxy evidence and implications for glacial atmospheric CO2

We present time series of export productivity proxy data including 230Thex-normalized deposition rates (rain rates) of 10Be, dissolution-corrected biogenic Ba, and biogenic opal as well as authigenic U concentrations which are complemented by rain rates of total (detrital) Fe and sea ice indicating diatom abundances from five sediment cores across the Atlantic sector of the Southern Ocean covering the past 150,000 years. The results suggest that 10Be rain rates and authigenic U concentration cannot serve as quantitative paleoproductivity proxies because they have also been influenced by detrital particle fluxes in the case of 10Be and bulk sedimentation rates (sediment focussing) and deep water oxygenation in the case of U. The combined results of the remaining productivity proxies of this study (rain rates of biogenic opal and biogenic Ba in those sections without authigenic U) and other previously published proxy data from the Southern Ocean (231Pa/230Th and nitrogen isotopes) suggest that a combination of sea ice cover, shallow remineralization depth, and stratification of the glacial water column south of the present position of the Antarctic Polar Front and possibly Fe fertilization north of it have been the main controlling factors of export paleoproductivity in the Southern Ocean over the last 150,000 years. An overall glacial increase of export paleoproductivity is not supported by the data, implying that bioproductivity variations in the Southern Ocean are unlikely to have contributed to the major glacial atmospheric CO2 drawdown observed in ice cores.

Low dissolved Fe and the absence of diatom blooms in remote Pacific waters of the Southern Ocean

The remote waters of the Pacific region of the Southern Ocean are the furthest away from any upstream and upwind continental Fe sources. This prime area for expecting Fe limitation of the plankton ecosystem was studied (March–April 1995) along a north–south transect at ∼89°W. At the end of the austral summer the upper wind-mixed layers were in the order of ∼100 m deep, thus mixing the algae down into the dimly lit part of the euphotic zone where photosynthesis is severely restricted. The dissolved Fe was found at low concentrations ranging from 0.05 nM near the surface to 0.5 nM in deeper waters. Along the transect (52°S–69°S), the dissolved iron was enhanced in the Polar Front, as well as near the Antarctic continental margin (0.6–1.0 nM). In between, the southern ACC branch was depleted with iron; here the concentrations in surface waters were quite uniform at about 0.21 nM. This is only somewhat lower than the 0.49 nM (October 1992) and 0.31 nM (November 1992) averages in early spring in the southern ACC part of Atlantic 6°W sections [de Baar, H.J.W., de Jong, J.T.M., Bakker, D.C.E.. Loscher, B.M., Veth, C., Bathmann, U., Smetacek, V., 1995. Importance of iron for phytoplankton spring blooms and CO2 drawdown in the Southern Ocean. Nature 373, 412–415; Loscher, B.M., de Jong, J.T.M., de Baar, H.J.W., Veth, C., Dehairs, F., 1997. The distribution of iron in the Antarctic Circumpolar Current. Deep-Sea Research II 44, 143–188.]. First, the lower ∼0.21 nM in March–April 1995 may partly be due to continuation of the seasonal trend where the phytoplankton growth, albeit modest, was removing Fe from the surface waters. Secondly, the 89°W Pacific stations are further downstream continental or seafloor sources than the Atlantic 6°W section. In the latter case, the ACC water had passed through the Drake Passage and also over the Sandwich Plateau. Indeed for Drake Passage, intermediate Fe concentrations have been reported by others. The generally somewhat lower surface water Fe at the ACC and PF at 89°W is consistent with the distance from sources and the late summer. It also would explain the very low abundance of phytoplankton (Chl a) in the region and the conspicuous absence of plankton blooms. In the subAntarctic waters north of the Polar Front there are no diatoms, let alone diatom blooms, due to low availability of silicate. Thus, it appears the biological productivity is suppressed due to iron deficiency, in combination with the severe seasonal effects of wind mixing on the light climate, as well as regional silicate limitation for diatoms.

Scavenging of 230Th and 231Pa near the antarctic polar front in the South Atlantic

Abstract Vertical profiles of dissolved and particulate 230 Th and 231 Pa were obtained across the Antarctic Circumpolar Current (ACC) in the southern Atlantic. North of the Polar Front, dissolved and total 230 Th increase with depth in conformity with published scavenging models. There is no depletion of 230 Th or 231 Pa in the water column south of the Polar Front, thought to be an area of enhanced biological productivity. 230 Th concentrations increase three-fold to the Weddell Sea across the ACC. Dissolved and total 231 Pa concentrations are relatively constant below 500 m depth at about 0.3 dpm m −3 , and change little with depth or latitude. The results from the Weddell Gyre are explained by a mixing-scavenging model that takes into account the input of lower Circumpolar Deep Water through upwelling, which is the main source of water in the Weddell Gyre and is enriched in 230 Th but not in 231 Pa. 230 Th accumulates in the Weddell Gyre as a result of a reduction in the scavenging rate and by ingrowth from 234 U. Ingrowth is more significant for 230 Th than for 231 Pa because the residence time of water in the gyre (about 35 years) is similar to the scavenging residence time of Th in the south Atlantic (29 years) but shorter than that of Pa (120 years). It is argued that changes in 230 Th accumulation in the past may reflect changes in water residence time and in the formation rate of Weddell Sea Deep Water.

Carbon export during the Spring Bloom at the Antarctic Polar Front, determined with the natural tracer 234Th

Profiles of particulate and dissolved 234Th were obtained during the JGOFS Southern Ocean expedition on R.V. Polarstern during October/November 1992. Measurements were made on three transects across the Antarctic Circumpolar Current, from the Polar Front (PF) in the north to the Weddell Sea/ACC boundary in the south. The dissolved 234Th238U ratio in surface waters gradually decreased during the development of the plankton bloom at the Polar Front. In the period between the first two transects, the 234Th activity removed from the dissolved phase had shifted to particles that had been produced, and as a result, the total activity ratio remained unchanged. The decrease in dissolved 234Th corresponds with decreases in dissolved nutrients and PC02, and with increases in chlorophyll and plankton biomass. Only during the third transect was the total activity of 234Th clearly reduced. The 234Th activity missing in the upper 100 m amounted to approximately 6 × 104 dpm m-2 at the Polar Front. In the 22-day period between the second and third transect, the 234Th export flux averaged 3200 dpm m−2 day−1. The ratio of organic carbon to 234Th on suspended particles was lower near the Polar Front than to the south, which we attribute to the higher abundance of empty diatom frustules at the Polar Front. With an average Corg/234Th ratio on suspended particles in surface water of about 20 μmol dpm−1, and assuming that the Corg/234 Th ratio on exported particles is 30–60% of this value, we estimate that 0.43–0.86 mol C m−2 had been removed over the 22-day period from the surface ocean by sinking particles. Export production was negligible in the Antarctic Zone including the ice edge, but during the later stage of the bloom in the Polar Front region, it amounted to 12–24% of primary production or 25–50% of the net CO2 uptake as estimated from a CO2 budget.

An intercomparison of small- and large-volume techniques for thorium-234 in seawater

Abstract In this paper, an intercomparison of methods for the determination of 234Th in seawater is discussed. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts in deep waters off Hawaii and the Southern Ocean. We compared large-volume techniques, which rely upon Mn cartridges for the collection of dissolved 234Th and its detection via gamma counting (>200-l samples), with small volume methods that employed either direct beta counting, or beta counting after radiochemical purification (2–20-l samples). Unique to this study is the presentation of small volume (2 and 5 l) 234Th methods. This new technique is an adaptation of 20-l methods that are based on the coprecipitation of thorium with Mn oxides followed by direct beta counting of the precipitate. The small volume Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection, processing and low overall analytical uncertainties.

A two-tracer (210Po–234Th) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current

We attempt to quantify and qualify the particle export from the surface water of the Antarctic Circumpolar Currentduring a spring phytoplankton bloom by the simultaneous use of the tracers 210Po and 234Th. We present data from theSouthern Ocean JGOFS expedition in 1992 at about 61W, from the marginal ice zone to the Polar Frontal region.Radionuclide export was calculated with a one-dimensional non-steady-state scavenging model. Rapidly changingactivities of 210Pb and 210Po during the phytoplankton bloom and the application of the tracer pair 210Po/210Pb withparticle-reactive parent and daughter required a new solution to the non-steady-state scavenging model. The observedfractionation of 210Po and 234Th on particles, dependent on particle composition (POC/biogenic silica ratio),corroborates the known preference of 210Po for cytoplasm. A combination of these two tracers can help to characterizethe nature (i.e. organic carbon and biogenic silica content) of the material settling out of the mixed layer, and thus toarrive at a more detailed interpretation of export fluxes than is possible with 234Th alone. In the Polar Front region,where diatoms are dominant in the phytoplankton and where the highest export rates were observed, we found apreferential settling of biogenic silica when heavily silicified diatom species occur. In contrast, POC and biogenic silicaare exported with comparable efficiency when diatom species with thinner frustules prevail. The export of biogenic opaland carbon is then closely coupled. In the southern Antarctic Circumpolar Current (sACC), where siliceous organismsare not dominant in the plankton, we found a preferential settling of siliceous material over POC.r2001 Elsevier ScienceLtd. All rights reserved.

Early diagenetic production and sediment-water exchange of fluorescent dissolved organic matter in the coastal environment

Abstract Fluorescence at wavelengths characteristic of humic substances (excitation 350 nm, emission 450 nm) have been used in this study to approximate concentrations of fluorescent dissolved organic material (FDOM). In situ regulated and unregulated benthic chambers, sediment cores, and laboratory tank incubations were used to study early diagenesis of FDOM in coastal marine sediments of the Gullmar Fjord, western Sweden. In the regulated in situ chambers, pH and oxygen were kept at relatively stable levels, while in the unregulated in situ chambers, pH and oxygen were left to decrease as a result of biological activity. FDOM porewater distributions and correlation between FDOM and parameters indicating mineralization showed that FDOM was formed in the sediment and should flux across the sediment-water interface. A substantial flux of FDOM was also observed during winter and spring conditions and during anoxic conditions fall. However, no flux was observed during oxic conditions fall. Modeling indicated that oxygen penetration depth was deeper during winter than during fall, i.e., the oxygen penetration depth increased during fall towards winter values. We suggest that as FeOOH was formed when oxygen penetration depths increased, FROM was sorbed to newly formed FeOOH, inhibiting FDOM flux over the sediment-water interface. In addition, at onset of anoxic conditions in the sediment surface layer in fall incubations, FDOM flux from sediment to overlying water increased substantially. Increases in anoxic FDOM fluxes were accompanied by increases in Fe and phosphate fluxes. We suggest that reductively dissolved FeOOH released sorbed FDOM. FDOM released from FeOOH by anoxic conditions was not resorbed when oxic conditions were resumed. This could be an effect of higher pH in overlying water as compared with porewater, inhibiting FeOOH sorption of FDOM.