David J. DeMaster

The Silica Balance in the World Ocean: A Reestimate

The net inputs of silicic acid (dissolved silica) to the world ocean have been revised to 6.1 � 2.0 teramoles of silicon per year (1 teramole = 1012 moles). The major contribution (about 80 percent) comes from rivers, whose world average silicic acid concentration is 150 micromolar. These inputs are reasonably balanced by the net ouputs of biogenic silica of 7.1 � 1.8 teramoles of silicon per year in modern marine sediments. The gross production of biogenic silica (the transformation of dissolved silicate to particulate skeletal material) in surface waters was estimated to be 240 � 40 teramoles of silicon per year, and the preservation ratio (opal accumulation in sediment/gross production in surface waters) averages 3 percent. In the world ocean the residence time of silicon, relative to total biological uptake in surface waters, is about 400 years.

A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy

Abstract Due to the major role played by diatoms in the biological pump of CO2, and to the presence of silica-rich sediments in areas that play a major role in air–sea CO2 exchange (e.g. the Southern Ocean and the Equatorial Pacific), opal has a strong potential as a proxy for paleoproductivity reconstructions. However, because of spatial variations in the biogenic silica preservation, and in the degree of coupling between the marine Si and C biogeochemical cycles, paleoreconstructions are not straitghtforward. A better calibration of this proxy in the modern ocean is required, which needs a good understanding of the mechanisms that control the Si cycle, in close relation to the carbon cycle. This review of the Si cycle in the modern ocean starts with the mechanisms that control the uptake of silicic acid (Si(OH)4) by diatoms and the subsequent silicification processes, the regulatory mechanisms of which are uncoupled. This has strong implications for the direct measurement in the field of the kinetics of Si(OH)4 uptake and diatom growth. It also strongly influences the Si:C ratio within diatoms, clearly linked to environmental conditions. Diatoms tend to dominate new production at marine ergoclines. At depth, they also succeed to form mats, which sedimentation is at the origin of laminated sediments and marine sapropels. The concentration of Si(OH)4 with respect to other macronutrients exerts a major influence on diatom dominance and on the rain ratio between siliceous and calcareous material, which severely impacts surface waters pCO2. A compilation of biogenic fluxes collected at about 40 sites by means of sediment traps also shows a remarkable pattern of increasing BSi:Corg ratio along the path of the “conveyor belt”, accompanying the relative enrichment of waters in Si compared to N and P. This observation suggests an extension of the Si pump model described by Dugdale and Wilkerson (Dugdale, R.C., Wilkerson, F.P., 1998. Understanding the eastern equatorial Pacific as a continuous new production system regulating on silicate. Nature 391, 270–273.), giving to Si(OH)4 a major role in the control of the rain ratio, which is of major importance in the global carbon cycle. The fate of the BSi produced in surface waters is then described, in relation to Corg, in terms of both dissolution and preservation mechanisms. Difficulties in quantifying the dissolution of biogenic silica in the water column as well as the sinking rates and forms of BSi to the deep, provide evidence for a major gap in our understanding of the mechanisms controlling the competition between retention in and export from surface waters. The relative influences of environmental conditions, seasonality, food web structure or aggregation are however explored. Quantitatively, assuming steady state, the measurements of the opal rain rate by means of sediment traps matches reasonably well those obtained by adding the recycling and burial fluxes in the underlying abyssal sediments, for most of the sites where such a comparison is possible. The major exception is the Southern Ocean where sediment focusing precludes the closing of mass balances. Focusing in fact is also an important aspect of the downward revision of the importance of Southern Ocean sediments in the global biogenic silica accumulation. Qualitatively, little is known about the duration of the transfer through the deep and the quality of the material that reaches the seabed, which is suggested to represent a major gap in our understanding of the processes governing the early diagenesis of BSi in sediments. The sediment composition (special emphasis on Al availability), the sedimentation rate or bioturbation are shown to exert an important control on the competition between dissolution and preservation of BSi in sediments. It is suggested that a primary control on the kinetic and thermodynamic properties of BSi dissolution, both in coastal and abyssal sediments, is exerted by water column processes, either occuring in surface waters during the formation of the frustules, or linked to the transfer of the particles through the water column, which duration may influence the quality of the biogenic rain. This highlights the importance of studying the factors controlling the degree of coupling between pelagic and benthic processes in various regions of the world ocean, and its consequences, not only in terms of benthic biology but also for the constitution of the sediment archive. The last section, first calls for the end of the “NPZD” models, and for the introduction of processes linked to the Si cycle, into models describing the phytoplankton cycles in surface waters and the early diagenesis of BSi in sediments. It also calls for the creation of an integrated 1-D diagnostic model of the Si:C coupling, for a better understanding of the interactions between surface waters, deep waters and the upper sedimentary column. The importance of Si(OH)4 in the control of the rain ratio and the improved parametrization of the Si cycle in the 1-D diagnostic models should lead to a reasonable incorporation of the Si cycle into 3-D regional circulation models and OGCMs, with important implications for climate change studies and paleoreconstructions at regional and global scale.

Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea

Abstract Radiochemical measurements of 234Th (t1/2 = 24days), 137Cs (bomb-produced), and 210Pb (t1/2 = 22y) have been used to characterize rates of mixing, deposition, and accumulation on 100-day and 100-y time scales in East China Sea sediments. In the inner-shelf mud deposit near the mouth of the Changjiang (Yangtze River), 234Th data indicate deposition rates as rapid as 4.4 cm month−1 on a 100-day time scale. 210Pb data indicate that on a 100-y time scale accumulation rates are an order of magnitude slower (typically 1 to 5 cm y−1) than the short-term deposition rates. Most of the sediment deposited near the mouth of the Changjiang on a 100-day time scale is transported southward along the dispersal system on a 100-y time scale, probably as a result of winter storms and a strong coastal current.210Pb accumulation rates from the inner-shelf mud deposit indicate that approximately 40% of the sediment discharged by the Changjiang can be accounted for in the sediments north of 30°N. The offshore mud deposit in the East China Sea is associated with the Huanghe (Yellow River) dispersal system. The dominant process affecting radionuclide profiles within this deposit is particle mixing (not sediment accumulation). In this area the upper 5 cm of the seabed are intensely reworked (Db = 26cm2 y−1) relative to the zone between 5 and 25 cm (Db = 2cm2 y−1). The maximum accumulation rate in the offshore mud deposit is 0.3 cm y−1. Less than 2% of the sediment discharged by the Huanghe can be accounted for in the offshore mud deposit. The relative intensity of mixing and accumulation is different for the proximal deposits of the Changjiang (where accumulation dominates) relative to the distal deposits of the Huanghe (where mixing dominates). Near the Changjiang mouth radiographs show distinct horizontal stratification, and the value of G (mixing rate/accumulation rate) is near zero. In the off-shore mud deposit radiographs reveal nearly homogeneous sedimentary structure, and the G value is ⩾18.

Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea

Sediment accumulation in the Yellow Sea epicontinental-shelf environment is investigated on 100-yr and 1000-yr time scales using 210Pb and 14C geochronologies. The distribution of modern (210Pb) accumulation rates in the Yellow Sea reveals that the loci of modern Huanghe sediment accumulation are the topset (1–2 mm/yr), foreset (4–9) mm/yr), and proximal bottomset deposits (2–4 mm/yr) of the Shandong subaqueous delta (which extends south from the Shandong Peninsula). 210Pb rates in the distal bottomset deposits of the subaqueous delta (in the central and southern Yellow Sea) are generally low (0.3–0.9 mm/yr). A sediment budget demonstrates that between 9–15% of the annual Huanghe discharge is accumulating in the Yellow Sea. About two-thirds of this sediment is accumulating in the topset, foreset and proximal bottomset deposits of the Shandong subaqueous delta, with the remaining third accumulating as widespread distal bottomset deposits. 14C age dates of subaqueous delta sediments indicate that the thick (∼40m) clinoform structure formed predominantly between 6200 and 4060 yrs B.P. (at rates approaching 20 mm/yr). Observations in the Yellow Sea, as well as on the Amazon and Ganges-Brahmaputra shelves demonstrate that subaqueous-deltaic stratigraphy is the general rule where major rivers enter energetic shelves, whether epicontinental or pericontinental. The development of extensive bottomset deposits may be restricted to epicontinental-shelf environments and may be diagnostic of sedimentation in this type of setting.

Nature of sediment accumulation on the Amazon continental shelf

Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g.210Pb,14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment.210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from 100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 108 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 108tons y−1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.

Rapid subduction of organic matter by maldanid polychaetes on the North Carolina slope

In situ tracer experiments conducted on the North Carolina continental slope reveal that tube-building worms (Polychaeta: Maldanidae) can, without ingestion, rapidly subduct freshly deposited, algal carbon ( 13 C-labeled diatoms) and inorganic materials (slope sediment and glass beads) to depths of 10 cm or more in the sediment column. Transport over 1.5 days appears to be nonselective but spatially patchy, creating localized, deep hotspots. As a result of this transport, relatively fresh organic matter becomes available soon after deposition to deep-dwelling microbes and other infauna, and both aerobic and anaerobic processes may be enhanced. Comparison of tracer subduction with estimates from a diffusive mixing model using 234 Th-based coefficients, suggests that maldanid subduction activities, within 1.5 d of particle deposition, could account for 25-100% of the mixing below 5 cm that occurs on 100-day time scales. Comparisons of community data from the North Carolina slope for different places and times indicate a correlation between the abundance of deep-dwelling maldanids and the abundance and the dwelling depth in the sediment column of other infauna. Pulsed inputs of organic matter occur frequently in margin environments and maldanid polychaetes are a common component of continental slope macrobenthos. Thus, the activities we observe are likely to be widespread and significant for chemical cycling (natural and anthropogenic materials) on the slope. We propose that species like maldanids, that rapidly redistribute labile organic matter within the seabed, probably function as keystone resource modifiers. They may exert a disproportionately strong influence (relative to their abundance) on the structure of infaunal communities and on the timing, location and nature of organic matter diagenesis and burial in continental margin sediments.

Phytodetritus at the abyssal seafloor across 10 of latitude in the central equatorial Pacific

Fresh phytoplankton detritus (or phytodetritus) has been reported from numerous deep seafloor sites in the temperate North Atlantic and Pacific Oceans following seasonal phytoplankton blooms. Here we report the first strong evidence for abyssal accumulations of phytodetritus in the tropics, in the central equatorial Pacific. In November–December 1992 we obtained photographs and/or sediment-core samples from 61 abyssal stations (water depths of 4280–5012 m) between 12°S and 9°N along ∼ 140°W. Greenish flocculent material was recovered from the top of multiple-core samples from 5°S to 5°N; this material was most abundant from 2°S to 2°N, in some areas forming continuous layers at least 5 mm thick, and individual aggregates > 1 cm in diameter. The greenish material was clearly visible in bottom photographs as a green veneer that covered >95% of the seafloor near the equator, and as individual cm-scale aggregates covering <1% of the seafloor. Occasionally, thick accumulations of cm-scale aggregates occurred in biogenic pits. Cleared trails and feeding traces suggest that surface-deposit-feeding holothurians and echiurans grazed the greenish material. Microscopic examination of greenish material recovered from core tops and a burrow lumen revealed relatively intact diatoms (including Rhizosolenia sp.) and other microalgae with chloroplasts containing chlorophyll. The greenish material was 1–12.5% organic carbon by weight, i.e. 5–39 times richer than associated seafloor sediments. It also contained high excess activities of 234Th, suggesting arrival from the water column in the previous 100 days. Samples of the greenish flocculent material from 0° and 5°N incubated at simulated environmental pressure and temperature with 14C-labeled glutamate exhibited ⩾ 5-fold higher rates of microbial activity than underlying sediments or brown floc from 9°N. Surface-sediment samples (which included the greenish flocculent material) from 5°S to 5°N also contained significant concentrations of chlorophyll a and other chloropigments; the chloropigment concentrations were roughly comparable to deep-sea phytodetritus collected in the North Atlantic. We conclude that fresh, organic-rich phytodetritus was present on the seafloor from 5°S to 5°N along 140°W in November–December 1992, with highest concentrations within 2–3° of the equator. This material is likely to be a concentrated, high-quality food resource for deep-sea microbes and metazoans. We estimate an upper limit for the standing stock of this phytodetritus to be ∼2.6 mmol Corg/m2; this corresponds to ∼3% of the annual flux of organic carbon to the seafloor at these latitudes in 1992. Because the degradation rate of this material appears to be very high, its presence at the seafloor for several months per year could yield significant phytodetrital contributions to the annual seafloor organic-carbon budget. We also suggest that the phytodetrital aggregates are formed at intense convergence zones resulting from seasonal passage of tropical instability waves within 5° of the equator; if so, phytodetrital accumulations are likely to recur seasonally over broad areas of the abyssal equatorial Pacific.

The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf

Abstract Nine cores from the Washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from Pb-210 profiles. Th-234 profiles indicate mixing coefficients of 140 cm 2 yr −1 for the seabed offshore from the Columbia River and 47 cm 2 yr −1 for the seabed of the Mid-Shelf Silt Deposit (75 km north of the Columbia River). These large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer (∼ 10 cm) within one year after emplacement at the seabed surface. Observed depths of Cs-137 penetration within the seabed are compared with depths predicted from the surface mixed-layer thickness and the Pb-210 accumulation rate. The observed and predicted depths agree well at all but one station. At this station a combination of Th-234, Co-60, Pb-210 and Cs-137 profiles suggests that active mixing occurs below the intensely mixed surface layer. The agreement of Pb-210 and Cs-137 data at the other stations, however, indicates a general absence of deep mixing on the Washington shelf. Thus, the apparent accumulation rates calculated from Pb-210 profiles (below the intensely mixed surface layer) on the Washington shelf generally reflect the true rates of sediment accumulation.

The deltaic nature of Amazon shelf sedimentation

Despite the annual discharge of more than a billion tons of sediment by the Amazon River, the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta. The river mouth, however, is not an estuary, either. These observations raise a major question as to what type of sedimentary environment the Amazon river mouth represents. Seismic stratigraphy has been examined on the continental shelf at the mouth of the Amazon River using high-frequency (3.5-kHz) seismic records from about 6,000 km of ship track. These records demonstrate three regions. (1) The Amazon River has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth. The feature is prograding seaward and accreting upward, and it contains fine-scale stratification typical of classic deltas. The feature forming at the mouth of the Amazon is a subaqueous delta; it differs from classic deltas primarily in its lack of subaerial expression. Subaqueous deltas, such as the Amazon, represent the general case of a major river entering an energetic oceanic regime.

Cycling of organic carbon and biogenic silica in the Southern Ocean: Estimates of water-column and sedimentary fluxes on the Ross Sea continental shelf

We examined the cycling of organic carbon and biogenic silica in the water column and upper sediments of the Ross Sea, seeking to understand the processes leading to the formation of opal-rich, organic-poor sediments over much of the Southern Ocean. Between January, 1990 and December, 1994 we conducted three cruises, performing tracer incubation studies (14C, 15N, 30Si, 32Si) to measure rates of primary production, nitrate-based “new” production, biogenic silica production and biogenic silica dissolution in the upper 50 m over most of the Ross Sea shelf in spring, mid summer and late summer. We deployed sediment traps from January, 1990 to early March, 1992 to measure the mid-water (250 m) and near-bottom gravitational fluxes of particulate organic carbon, nitrogen and biogenic silica year-round at three sites, and obtained sediment cores at 15 sites to assess the accumulation rates of organic carbon and biogenic silica in all known sediment regimes on the shelf. At 9 of those sites we also measured nutrient efflux from the sediments, enabling us to calculate benthic recycling fluxes of organic matter and opal. These data permit estimates of the annual production, near-surface recycling, vertical sinking flux, delivery to the seabed, benthic regeneration and long-term burial of both organic and siliceous material, integrated over a 3.3 × 105 km2 area that covers 75–80% of the Ross Sea shelf. The resulting annual budgets for carbon and silica indicate highly selective preservation of biogenic silica over organic carbon between 50 and 250 m in the water column, as well as in the upper seabed. Selective preservation of silica within the upper 50 m is not indicated, and both organic matter and silica are transported from 250 m to the sea floor with virtually 100% efficiency. The SiO2/C mass ratios for surface-layer production, 250-m sinking flux, delivery to the seabed and long-term burial are approximately 0.85, 6.1, 6.2 and 27, respectively. This progressive enrichment in silica results in long-term burial of 5.8% of the biogenic silica and 0.17% of the organic carbon produced by phytoplankton in the surface layer, a factor of 30 greater preservation efficiency for silica than for carbon. Nevertheless, the ratio of opal burial to opal production in the Ross Sea is only about twice the apparent global average of 3% and <1/3 of the estimated burial/production ratio for the Southern Ocean as a whole. It thus appears that both silica preservation and the decoupling between the cycles of silica and carbon must be even more effective in the waters overlying abyssal Southern Ocean sediments than they are over the Ross Sea shelf.