Biological Production Rates Research Articles

What Controls Opal Preservation in Tropical Deep‐Sea Sediments?

Measurements of opal preservation in deep sea sediment cores have been presented in three ways: the opal concentration as a fraction of total dry weight (%opaltot), the opal concentration normalized to calcite‐free dry weight (%opalcalcite‐free), and me opal accumulation rate (opal MAR). It is tempting to interpret changes in these indices as indicators of rates of biological production in past oceans. Based on theoretical constraints, we argue that in typical tropical and subtropical sediments, both %Opalcalcite‐free and opal MAR reflect a significant artifact of dilution by other phases. Thus the band of high %Opalcalcite‐free in the equatorial Pacific appears to be caused in large part by the high %Calcite in that region, rather than by high opal productivity. The best candidate for a reliable paleoproductivity proxy appears to be %Opaltot. Unfortunately, present‐day %Opaltot data from tropical and subtropical regions show little or no systematic trend with the rain rate of opal. Pore water silica concentration data reveal that the apparent pore water opal solubility is not constant but correlates regionally with the rain rate of opal to the seafloor. A model that treats opal as a single homogeneous phase with a single well‐defined solubility product predicts a strong dependence of opal concentration on rain rate (in stark contrast to the data), and a constant asymptotic pore water Si. Two models representing opal as multiple heterogeneous phases with different solubilities are able to reproduce the observed asymptotic pore water Si/rain rate relationship, but not the lack of rain rate trend in the opal concentration data. Only by assuming a systematic trend in the quality of opal (i.e., the solubility) as a function of opal production, can we reproduce the observed pattern of opal preservation. The implication of this study is that changes in opal preservation in the geologic record cannot simply be interpreted in terms of changes in surface ocean productivity until our understanding of opal diagenesis can be improved.

Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle

Measurements of changes in atmospheric molecular oxygen using a new interferometric technique show that the O2 content of air varies seasonally in both the Northern and Southern Hemispheres and is decreasing from year to year. The seasonal variations provide a new basis for estimating global rates of biological organic carbon production in the ocean, and the interannual decrease constrains estimates of the rate of anthropogenic CO2 uptake by the oceans.

Nitrogenous nutrition of sea-ice microalgae

There are indications that the final biomass attained by sea-ice microalgae in southeastern Hudson Bay is nitrogen limited. The present study investigates the possibility that the rate at which the final yield is approached is also nitrogen limited. Nutrient data suggest that nitrogen was actively taken up by the microalgae, and periodically replenished by mixing processes related to fortnightly tides. High molar C:N ratios (>15), typical of nitrogen-deficient cells, have been observed at times of low dissolved inorganic nitrogen. Absolute transport rates of nitrogen were estimated from natural changes in dissolved inorganic and particulate organic nitrogen. Cell losses and nitrogen uptake rates were highest during periods of maximum current speed, suggesting that the rate of biological production at the ice-water interface could be limited by the accumulated biomass. These results suggest (1) that the sea-ice microalgae in southeastern Hudson Bay are nitrogen limited in their natural environment, and (2) that nutrient replenishment and perhaps losses of biomass governed by fortnightly tidal mixing periodically enhance the growth of microalgae at the ice-water interface.

A comparison of soil climate and biological activity along an elevation gradient in the eastern Mojave Desert.

Soil temperature, moisture, and CO2 were monitored at four sites along an elevation transect in the eastern Mojave Desert from January to October, 1987. Climate appeared to be the major factor controlling CO2 partial pressures, primarily through its influence of rates of biological reactions, vegetation densities, and organic matter production. With increasing elevation, and increasing actual evapotranspiration, the organic C, plant density, and the CO2 content of the soils increased. Between January and May, soil CO2 concentrations at a given site were closely related to variations in soil temperature. In July and October, temperatures had little effect on CO2, presumably due to low soil moisture levels. Up to 75% of litter placed in the field in March was lost by October whereas, for the 3 lower elevations, less than 10% of the litter placed in the field in April was lost through decomposition processes.

Latest Cretaceous-Early Neogene oxygen and carbon isotopic record at DSDP sites in the Indian Ocean

Stable isotopic data of calcareous nannofossil, monogeneric and monospecific planktic and benthic foraminifera from five Indian Ocean DSDP sites (212, 217, 220, 237, and 253), leads to the following paleoclimatic and paleoceanographic conclusions: • — The latest Cretaceous oxygen isotopic record implies a cooling (3–4°C) during the Maastrichtian. At the Cretaceous/Tertiary boundary only a minor warming (about 2°C) has been recorded. The parallel δ 13 C decrease of more than 1‰ indicates a significant decrease in productivity. • — During the latest Paleocene a positive δ 13 C excursion was detected in Sites 217 and 237. This transient enrichment in δ 13 C may be due to productivity changes on continents and/or a change in the storage rate of organic matter in marginal basins or shelf areas. • — The most striking feature in the oxygen isotopic record is noted at the Early/Middle Eocene transition. The shift towards more positive values (which were probably enhanced to a certain extent by a preceding diagenetic alteration) delineates a dramatic climatic deterioration at high and mid latitudes during the earlier Tertiary. • — Near the Eocene/Oligocene boundary a cooling is evident within the latest Eocene interval. During the earliest Oligocene time a hiatus at Sites 217 and 253 partially obscures the climatic record. • — Several climatic fluctuations have been noted during the Oligocene: a cooling at the base of Zone NP 23, a warming at the top of Zone NP 23 through NP 24, and a cooling during Zone NP 25. • — The Miocene oxygen isotopic record is dominated by changes in surface and bottom water environments during Zone NN5. The decreasing and then increasing δ 18 O values, together with the subsequent steepening of the vertical δ 18 O gradient, point towards major climatic instabilities. These events coincide with the Mid-Miocene build-up of Antarctic ice-sheets. During the latest Miocene to the earliest Pliocene the δ 18 O record of planktic foraminifera indicates a significant warming of the Indian Ocean at mid-latitudes. • — The δ 13 C record during the Oligocene and Miocene reveals several cycles ( δ 13 C enrichments: NP 24, NN2, NN5, NN9, and base NN 11) which are most likely related to changes in storage rates of organic matter and biological productivity due to climatic changes and transgression/regression cycles. In addition, changes in the circulation patterns may also have influenced the carbon isotopic record.

Diurnal Patterns and Regulation of Cortisol Secretion in Pregnancy*

Plasma concentrations of cortisol and corticosteroid-binding globulin are elevated substantially in pregnancy; only the free cortisol fraction is biologically active. Previous suggestions that plasma free cortisol increased above nongravid levels in pregnancy were difficult to reconcile with the absence of manifestations of hypercortisolism. Furthermore, any such elevation should be associated with some alteration in the regulation of cortisol secretion. Therefore, the physiology of cortisol secretion was studied by measuring plasma cortisol at 20-min intervals for 24 h. Free cortisol levels were evaluated by determining the plasma free cortisol index and the rate of urinary cortisol excretion. Circadian cortisol patterns in pregnant and nonpregnant subjects were virtually identical, but total plasma cortisol was significantly higher in pregnancy. Calculated plasma biological half-lives and production rates of cortisol were increased significantly in gravidas (t½: pregnant, 104.6 min; nonpregnant, 69.5 min; daily production rate: pregnant, 28.0 mg; nonpregnant, 11.2 mg). Such increases should produce rises in plasma free cortisol, an assumption supported by finding elevations of the free cortisol index in gravidas at 0800 and 2400 h (pregnant, 14.9 ± 1.9 and 3.3 ± 0.2; nonpregnant, 4.0 ± 0.6 and 1.2 ± 0.3) and elevations of the 24-h urinary cortisol excretion rate. Normal circadian rhythmicity despite elevated free cortisol suggests that maternal hypothalamic-pituitary regulation is altered. Resetting of feedback and lack of manifestations of hypercortisolism could be explained by tissue refractoriness to the effects of cortisol in pregnancy.