William J. Showers

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.

Oxygen isotope partitioning between phosphate and carbonate in mammalian apatite

Abstract The oxygen isotope compositions of phosphate and structural carbonate in mammalian enamel and bone apatite are linked to that of body water at constant body temperature near 37°C, but the isotope systematics of oxygen in structural carbonate are not well understood. Using coupled measurements of the oxygen isotope composition of structural carbonate and phosphate from horse tooth enamel, the apparent oxygen isotope fractionation factor between structural carbonate and body water is estimated to be 1.0263 ± 0.0014. These estimates provide a quantitative basis for using the oxygen isotope composition of structural carbonate in mammalian biogenic apatite for ecological, climatological, and physiological reconstruction.

Indicators of methane-derived carbonates and chemosynthetic organic carbon deposits: examples from the Florida Escarpment

Abyssal chemosynthetic communities are supported by bacterial oxidation of reduced chemicals in brines which seep out through sediments at the base of the Florida Escarpment. They are surrounded by carbonate hardgrounds and sediments rich in fresh organic carbon that contain a record of the metabolic pathways and geochemical processes which are active at these sites. The isotopic composition of tissue samples (δ 13 C as low as #7576.40∓), carbonate crusts (δ 13 C as low #7545.19∓) and sedimentary organic matter (δ 13 C as low #7567.87∓) indicate that biogenic methane dissolved in the brines (δ 13 C #7583.3 ± 5.8∓) is a major carbon source for many of the locally synthesized compounds

Biologic and climatic signals in the oxygen isotopic composition of Eocene-Oligocene equid enamel phosphate

Abstract Oxygen isotope results for tooth enamel-phosphate (δP) from late Eocene-early Oligocene fossil horses are presented to determine if paleobiologic and paleoclimatologic information is recorded in fossil tooth enamel chemistry. Teeth from jaws of Mesohippus and Miohippus from the White River Formation (or Group) in the western Great Plains are well preserved and have excellent geochronologic control. Although there is clear evidence for post-depositional alteration of the enamel, a hydroxylapatite mineralogy is preserved and isotopic exchange of oxygen does not appear to be significant. There are distinctive patterns of δP variation among individual teeth from the same jaw. These patterns reflect season of birth and timing of enamel mineralization. Most of the horses were born in the spring, and mineralization of the enamel is complete after 1–1.5 years. These results show that paleoclimate reconstruction from enamel δP must account for tooth position and timing of mineralization. The observation that there are different intrajaw patterns of δP variation among Mesohippus and Miohippus may provide a basis to reconstruct changes in climate seasonality in ancient environments.

Stable isotopic characterization of organic carbon accumulation on the Amazon continental shelf

The stable carbon isotopic ratio (13C:12C) of organic matter in the water column and sediments has been used to examine the distribution and accumulation of organic carbon on the Amazon continental shelf. Near the river mouth, in 0 × 10−3 salinity surface waters, particulate organic carbon (POC) is isotopically light (−27.3 per mil). Isotopic values of POC in continental shelf waters north and east of the river mouth range from −19.5 to −25.7 per mil. The isotopic variations in water column POC can be related to productivity, turbidity, and water density. The isotopic character of water column POC is controlled, therefore, by the dynamic mixing and northwestward migration of riverine and marine shelf waters. Terrestrial organic carbon dominates the isotopic signal in surface sediments (upper 10 cm) from the river mouth to areas 400 km to the northwest. Only on the outermost and northern parts of the shelf is marine organic carbon dominant in surface sediments. A sharp boundary between terrestrial and marine isotopic values is present in continental shelf sediments at the outer edge of the prograding mud delta. This boundary is associated with an abrupt decrease in sediment accumulation rate and a change in stratigraphic setting from topset and foreset regions to bottomset regions. The average TOC content of Amazon continental shelf sediments is 0.66 ± 0.20%. Based on the average TOC of Amazon shelf sediments and current estimates of sediment accumulation rates, approximately 4.5 × 1012 g-TOC y−1 is accumulating in Amazon shelf sediments. Using δ13C-POC values at salinities of 0 and 35 × 10−3 as end-members to isotopically resolve the contribution of terrestrial and marine carbon sources, approximately 3.1 × 1012 g-TOC y−1 or 69% of the organic carbon accumulating in Amazon shelf surface sediments is from terrestrial sources. This represents 6% of current estimates for Amazon riverine carbon discharge.

Thermophysiology of Tyrannosaurus rex: Evidence from Oxygen Isotopes

The oxygen isotopic composition of vertebrate bone phosphate (δp) is related to ingested water and to the body temperature at which the bone forms. The δp is in equilibrium with the individuals body water, which is at a physiological steady state throughout the body. Therefore, intrabone temperature variation and the mean interbone temperature differences of well-preserved fossil vertebrates can be determined from the δp variation. Values of δp from a well-preserved Tyrannosaurus rex suggest that this species maintained homeothermy with less than 4�C of variability in body temperature. Maintenance of homeothermy implies a relatively high metabolic rate that is similar to that of endotherms.

Methane transport mechanisms and isotopic fractionation in emergent macrophytes of an Alaskan tundra lake

The carbon isotopic composition of methane emitted by the Alaskan emergent aquatic plants Arctophila fulva, a tundra mid-lake macrophyte, and Carex rostrata, a tundra lake margin macrophyte, was −58.6 ± 0.5 (n=2) and −66.6±2.5 (n= 6) ‰ respectively. The methane emitted by these species was found to be depleted in 13C by 12‰ and 18‰, relative to methane withdrawn from plant stems 1 to 2 cm below the waterline. As the macrophyte-mediated methane flux represented approximately 97% of the flux from these sites, these results suggest the more rapid transport of 12CH4 relative to 13CH4 through plants to the atmosphere. This preferential release of the light isotope of methane, possibly combined with CH4 oxidation, caused the buildup of the heavy isotope within plant stems. Plant stem methane concentrations ranged from 0.2 to 4.0% ( x¯, 1.4; standard deviation (sd), 0.9; n=28) in Arctophila, with an isotopic composition of −46.1±4.3 ‰ (n = 8). Carex stem methane concentrations were lower, ranging from 150 to 1200 ppm ( x¯, 500; standard deviation, 360; n = 8), with an isotopic composition of −48.3±1.4‰ (n=3). Comparisons of the observed isotopic fractionations with those predicted from gas phase effusion and diffusion coefficients suggest a combination of one or both of these gas transport mechanisms with bulk (non-fractionationating) flow.

Carbon and hydrogen isotopic characterization of methane from wetlands and lakes of the Yukon-Kuskokwim Delta, western Alaska

The total methane flux to the troposphere from tundra environments of the Yukon-Kuskokwim Delta is dominated by emissions from wet meadow tundra (∼75%) and small, organic-rich lakes (∼20%). The mean δ13C value of methane diffusing into collar-mounted flux chambers from wet meadow environments near Bethel, Alaska, was −65.82 ± 2.21‰ (±1 sigma, n = 18) for the period July 10 to August 10, 1988. Detritus-rich sediments of Delta lakes, including margins of large lakes and entire submerged areas of smaller ones, are laden with gas bubbles whose methane concentration ranges from 11% to 79%. Lowest methane concentrations are found along heavily vegetated lake edge environments and highest through-out organic-rich, fibrous sediments of small lakes. A minimum ebullition flux estimated for the 5% of total Delta area comprised of small lakes ranges from 0.34 to 9.7 × 1010 g CH4 yr−1, which represents 0.6% to 17% of the total Delta methane emission. The δ13C and δD values of this ebullitive flux are −61.41 ± 2.46‰ (n = 38) and −341.8 ± 18.2‰ (n = 21), respectively. The methane in gas bubbles from two lakes is of modern, bomb carbon enriched, radiocarbon age. Gas bubble δ13C values varied from 2 to 5‰ seasonally, reaching heaviest values in midsummer, no such variations in δD values were observed. Combined isotope data reveal that higher δ13C values in heavily vegetated areas correlate with lower δD values, suggesting enhanced methane production via acetate fermentation. Spatial isotopic variations in lakes appear to be controlled by variations in production rather than oxidation processes.

Stable isotopes as tracers of methane dynamics in Everglades marshes with and without active populations of methane oxidizing bacteria

Methane flux from Cladium jamaicense varied from 0.2 to 15 mmol m−2 d−1 and was 1.4 to 26 (avg = 5.64 ± 8.57, n = 13, error is ± 1 standard deviation throughout) times greater than the flux from the flood water. The lack of diurnal variations in both the rate of CH4 emission and its stable carbon isotopic composition suggests that CH4 flux from Cladium was independent of stomatal aperture and that gases were transported through the plant mainly via passive diffusion and/or effusion as opposed to active pressurized ventilation. Rhizospheric CH4 oxidation did not cause 13C-enriched CH4 to be emitted to the atmosphere by Cladium jamaicense. Previous workers have shown that Everglades soil types differ in that CH4 oxidizing bacteria are active in peat soils and inactive in marl soils (King et al., 1990; Gerard, 1992), however a comparison of the stable isotopic composition of emitted and sedimentary CH4 from Cladium marshes within marl and peat soils provided no evidence that rhizospheric CH4 oxidizing bacteria were consuming significant quantities of CH4 in situ within peat soils. Either CH4 oxidation in the rhizosphere was insignificant due to O2 limitation or it occurred quantitatively in discrete zones within the sediment, thereby imparting no isotopic signal to sedimentary CH4. Linear relationships between CH4 flux and live aboveground Cladium biomass in marl and peat soils were identical and offered no evidence for rhizospheric CH4 oxidation in peat soils. In contrast core incubation experiments indicated that CH4 oxidizing bacteria at the sediment-water interface in peat soils intercepted and oxidized from 41 to 93 % (avg = 71 ± 20 %, n = 9) of the CH4 diffusing from the sediments toward the overlying flood water. Furthermore, we were able to detect sediment-water interface oxidation with stable isotopes as CH4 emitted from the flood water (δ13C = 57.3 ± 3.6 ‰, n = 5) after plants were clipped below the water surface was enriched in 13C by over 10 ‰ relative to CH4 emitted from vegetated plots (δ13C = −68.1 ± 2.5 ‰, n = 10). Methane within flood water (before clipping) at peat sites was also 13C enriched (δ13C = −57.6 ± 4.3 ‰, n = 7). Lowering of the water table below the sediment surface caused an Everglades sawgrass marsh to shift from CH4 emission to the consumption of atmospheric CH4 at a rate of 55 ± 41 μmol m−2d−1.

Paleoclimate of the past 4000 years at Lake Turkana, Kenya, based on the isotopic composition of authigenic calcite

Abstract Authigenic calcite was analyzed for δ 13 C and δ 18 O in a 12 m long core from Lake Turkana that spans the last 4200 yr. The δ 18 O record is interpreted to reflect lake level history, and indicates high stands prior to 4000 yr B.P., from 1800 to 2000 yr B.P., and within the last century. This coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for Lake Turkana based on 14 C dates of exposed shoreline deposits. The δ 13 C curve roughly parallels the δ 18 O curve, and may reflect the influence of the Omo River plume. Both of the isotope records are strongly cyclic, with periodicites common to both at 80, 120, 150 and 225 yr. Other, somewhat longer periodicites are nearly equal in the two records, and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same Turkana core.