Peter R. Betzer

Primary productivity and particle fluxes on a transect of the equator at 153°W in the Pacific Ocean

Abstract Primary productivity (14C) and mass flux measurements using a free-drifting sediment trap deployed at 900 m were made at four stations in the Pacific Ocean between 12°N and 6°S at 153°W. The latitudinal variations in productivity were consistent with historical patterns showing the equator as a zone of high production and the oligotrophic waters north of the equatorial region as an area of low productivity. The correlation coefficient between the two sets of independent measurements was 0.999, indicating that in this oceanic area the activity of the primary producers was closely related to the total mass flux. A re-examination of historical data suggests that the downward flux of particulate organic carbon varies in direct proportion to the quotient of surface primary production raised to the 1.4 power and depth raised to the 0.63 power.

Sediment resuspension by coastal waters: a potential mechanism for nutrient re-cycling on the ocean's margins

Abstract Nutrient profiles from the continental shelf of the northeastern Gulf of Mexico indicated considerable near-bottom enrichment in silica and nitrate above coarse sediments east of the Mississippi delta. In contrast, near-bottom water of the carbonate-rich West Florida Shelf showed no such enrichmmets. Storm-related suspension apparently produced the enrichments because, in near-bottom waters south of Mobile Bay, silica, nitrate plus nitrite, and suspended load increased subbtantially as a winter storm front passed. Also, laboratory simulation of resuspension by stirring the supernatant seawater over a clay-rich core produced similar increase in silica and nitrate plus nitrite, with ammonia being the apparent precursor to the nitrate and nitrite. Most of the nutrient increase appeared to come from previously deposited sediments in the early stages of resuspension. Using the ratios of nutrients released to sediments resuspended, calculations indicate that resuspension of as little as 1 mm of shelf sediment could intermittenly augment overlying productivity by as much as 100 to 200%. Thus, resuspension may accelerate nutrient recycling on continental margins.

Dissolution and Analysis of Amorphous Silica in Marine Sediments

ABSTRACT The analytical estimation of amorphous silica in selected Atlantic and Antarctic Ocean sediments, the U.S.G.S. standard marine mud (MAG-1), A.A.P.G. clays, and samples from cultures of a marine diatom, Hemidiscus, has been examined. Quantitative recovery of sedimentary amorphous silica was achieved by reacting 2 M Na2CO3 with a sample for 4 hours at 90°-100° C; where necessary, aluminum analysis is used to correct for the extraction of non-amorphous silica. Oceanic sediments having an amorphous SiO2/clay ratio of 1.0 or more can be analyzed by a single extraction with 2 M Na2CO3, without correction for non-amorphous additions. Marine sediments having an amorphous SiO2/clay ratio of from 1.0 to 0.25 can be corrected for inputs of clay-derived silica using an aluminum determination and an empirical correction factor. Sediments with amorphous SiO2/clay ratios of less than 0.25, samples containing non-silica-bearing aluminous materials such as gibbsite, or materials having low absolute levels of amorphous silica require successive leaches to accurately correct for silica inputs from non-amorphous sources. Our values for amorphous silica-rich circum-Antarctic sediments are equal to or greater than literature values, whereas our values for a set of amorphous silica-poor sediments from a transect of the North Atlantic at 11°N, after appropriate correction for silica released from clays, are significantly lower than previous estimates from the same region.

Particles fluxes and moving fluids: experience from synchronous trap collection in the Sargassso sea

A series of synchronous, 24-h experiments using sensor-equipped sediment traps revealed that higher particle collection rates were associated with higher approach velocieties. Surface-tethered traps with variable drag configurations provided distinct differences in approach velocities for paired 400 m deployments and paired 1500 m deployments. Small-scale hot-film hydrodynamics sensors located both inside and outside the sediment traps detected flow cells within the traps with velocities between 50 and 100% of the external fluid approach velocities. In conjunction with laboratory flume simulations, these observations reveal that particles do not settle gravitationally across trap apertures. Intead, particles are swept advectively into traps at the downstream portion of trap apertures, and most are then expelled at the upstream portions of trap apertures. Fluid flows detected inside the drifting traps, which ranged from 1.2 to 31 cm s−1,l probably overwhelm all but the strongest “swimmers” that interact with these sampling divices. At our two sampling horizons (400 and 1500 m), tether-line motions generated trap depth oscillations with a period of the order of 10 s and an amplitude of about 0.5 m. Such effects have not been accounted for in flume simulated of sediment traps collection experiments.

Geothermal springs of the West Florida continental shelf: Evidence for dolomitization and radionuclide enrichment

Abstract On the sea bed of the West Florida continental shelf about 45 km SSW of Ft. Myers, Florida, an 85-km 2 area has been discovered in which six thermal springs discharge warm, chemically altered seawater from vents and seepage zones. The spring water apparently originates in the subsurface ocean around the Florida Platform and penetrates the highly porous strata of the platform about 500–1000 meters below sea level. It percolates toward the interior of the platform and is geothermally heated to about 40°C en route. Then it rises along more vertical flow channels and is discharged in warm submarine springs. Beneath the platform, several chemical processes alter the percolating seawater. One process seems to be a secondary dolomitization of the limestone of the platform because, in the discharging seawater, magnesium is lower by 2.7 mmole/kg and calcium higher by 3.6 mmole/kg than in normal seawater with the same chlorinity. Other reactions within the sediments of the platform enrich the spring effluents 1000-fold in 226 Ra, 10,000-fold in 222 Rn, and 90-fold in 228 Ra compared to the seawater surrounding the platform. Thus, the springs may be important sources of radionuclides for the Gulf of Mexico. The percolating seawater also loses all of its oxygen and nitrate to reduction processes, loses most of its phosphate and 40% of its 238 U, and roughly quadruples its silica content. Coastal carbonate platforms are fairly common geological features. Thus, processes like those beneath the West Florida Shelf may function on a world-wide basis to play an important role in the diagenesis of carbonate sediments.

Winter-summer variations of calcite and aragonite saturation in the Northeast Pacific

Abstract New carbonate data obtained on February-March and June-July cruises in the northeast Pacific during 1985 were utilized to describe processes affecting seasonal variations of calcite and aragonite saturation. Large gradients in saturation state occur in the region between the Subtropical and the Subarctic Fronts in the north-south direction. These gradients are a function of largescale mixing and biological processes in the North Pacific. The saturation values in the upper kilometer of the water column were observed to be significantly lower in winter than in summer. These decreases were due to a number of processes including: (i) the seasonal decrease in the temperature of the water column; (ii) the seasonal increase in vertical mixing causing CO 2 -enriched deep waters to be upwelled; and (iii) the seasonal enhancement of respiration over photosynthesis. Seasonal changes in total CO 2 (TCO 2 ) concentrations appeared to have the greatest overall effect on the saturation state. Aragonite dissolution rate experiments were conducted during the June-July cruise to provide an independent verification of the saturation calculations. In all cases, significant increases in aragonite dissolution were observed below the 100% saturation depth. In the northeast Pacific, shallow undersaturation horizons provide for significant dissolution rates at depths below 400 m.

Physical, Chemical, and Optical Measures of Suspended-Particle Concentrations: Their Intercomparison and Application to the West African Shelf

A property of oceanic particulate matter referred to as “apparent density” was calculated by dividing the weight of suspended particulate matter CSPM) by the volume of particles. This parameter is equal to “mass density” for particles, such as minerals, containing little water. Apparent density calculations were made for a series of samples collected on R/V Trident cruise 112 to the continental shelves of Sierra Leone and Liberia. These values ranged from 0.104 to 1.79 for samples with particulate organic carbon fractions (POC/SPM) ranging from 0.486 to 0.037.

Metal-enriched sediments from the TAG Hydrothermal Field

A SERIES of sediment samples from the TAG Hydrothermal Field1 have been collected and analysed for transition metals, and for uranium and thorium isotopes. We show here that metal enrichment of sediment samples is ubiquitous, but geographically variable. Non-detrital metal accumulation rates in one core are Fe and Mn 15.0 and 0.7 mg cm−2 10−3 yr; and Ni, 180; Co, 93; Cu, 134; and Cr, 148 µg cm−2 10−3yr. The hydrothermal field is located on the eastern wall of the Mid-Atlantic Ridge Rift Valley at 26°N and covers ∼100 km2. It is characterised by extensive deposits of thick, rapidly growing MnO2 (ref. 2) and by a magnetic low in the axial anomaly1.

Concentrations of cadmium, copper, lead and zinc in surface waters of the Northwest Atlantic Ocean — A comparison of Go-flo and teflon water samplers

Abstract The concentrations of cadmium, copper, lead and zinc were measured at two locations in surface waters of the western North Atlantic. Samples were collected upstream of the research vessel in 1-1 teflon bottles, and in a 30-1 teflon-coated Go-flo bottle. Surface mixed-layer samples were also collected at each location by ship cast using this same Go-flo sampler. A comparison of the three sampling modes shows that all the samples taken with the Go-flo sampler possessed much higher concentrations of zinc (7–10-fold) and lead (2–3-fold) than those collected directly in teflon bottles. No apparent differences were noted at each station in the concentrations of either copper or cadmium among the samples collected by the three different procedures. The measured values for copper and cadmium in these waters are in good agreement with recent reports for the western North Atlantic.

The effect of pressure on aragonite dissolution rates in seawater

Abstract Aragonite dissolution in seawater at variable pressure is well described by the equation Rate = k′([CO2−3]s - [CO2−3])n where [CO2−3]s, is the carbonate ion concentration at saturation, [CO2−3] is the observed carbonate ion concentration and k′ and n are empirical constants. Application of the equation Rate = k(1 − Ω) n to descriptions of aragonite dissolution rates (where Ω = [CO 2− 3 ] [CO 2− 3 ] s ) is appropriate at constant pressure, temperature and salinity. Our measurements in seawater at 5°C are consistent with the estimate −ΔV = 36.5 cm3/mole for the volume change accompanying aragonite dissolution. This result is somewhat higher than laboratory estimates, but lower than estimates based on calcite and the difference between calcite and aragonite molar volumes.