W D Rugh

Deep-dwelling planktonic foraminifera of the northeastern Pacific Ocean reveal environmental control of oxygen and carbon isotopic disequilibria

Abstract We assess the utility of four species of living planktonic foraminifera as tracers of thermocline and intermediate water masses in the northern Pacific Ocean, based on their water-column distribution and shell isotopic composition. Assuming oxygen isotopic equilibria with the water column, we infer apparent calcification depths. This allows an estimate of apparent carbon isotopic disequilibria. We then relax the assumption of oxygen isotopic equilibrium to examine habitat effects on kinetic disequilibrium processes. In the California Current, left-coiling Neogloboquadrina pachyderma and Globigerina calida calcify in the upper thermocline. Globorotalia scitula resides within the Shallow Salinity Minumum (potential density σ θ = 25.1–26.7), while Globoquadrina hexagona is associated with the deeper, North Pacific Intermediate Water (σ θ = 26.7–26.9). Apparent carbon isotopic disequilibria corrected for oxygen isotopic disequilibria, range from 1.0–1.9‰ in these asymbiotic species. The carbon isotopic disequilibrium can be modeled as a single exponential function of temperature or a logarithmic function of potential food supply. We infer that carbon isotopic disequilibrium increases with metabolic rate, related to temperature and/or food supply. Kinetic processes of oxygen and carbon isotopic disequilibria yields reasonable depth habitats if the slope of the oxygen:carbon isotopic shift is about 0.35, consistent with culture data. Our finding of a link between environment, metabolism, and isotopic disequilibrium observed in oceanic settings suggests the potential to better reconstruct the structure and biological processes of the upper water column from geologic data.

USING NOCTURNAL COLD AIR DRAINAGE FLOW TO MONITOR ECOSYSTEM PROCESSES IN COMPLEX TERRAIN

This paper presents initial investigations of a new approach to monitor ecosystem processes in complex terrain on large scales. Metabolic processes in mountainous ecosystems are poorly represented in current ecosystem monitoring campaigns because the methods used for monitoring metabolism at the ecosystem scale (e.g., eddy covariance) require flat study sites. Our goal was to investigate the potential for using nocturnal down-valley winds (cold air drainage) for monitoring ecosystem processes in mountainous terrain from two perspectives: measurements of the isotopic composition of ecosystem-respired CO2 (delta13C(ER)) and estimates of fluxes of CO2 transported in the drainage flow. To test if this approach is plausible, we monitored the wind patterns, CO2 concentrations, and the carbon isotopic composition of the air as it exited the base of a young (approximately 40 yr-old) and an old (>450 yr-old) steeply sided Douglas-fir watershed. Nocturnal cold air drainage within these watersheds was strong, deep, and occurred on more than 80% of summer nights. The depth of cold air drainage rapidly increased to tower height or greater when the net radiation at the top of the tower approached zero. The carbon isotope composition of CO2 in the drainage system holds promise as an indicator of variation in basin-scale physiological processes. Although there was little vertical variation in CO2 concentration at any point in time, we found that the range of CO2 concentration over a single evening was sufficient to estimate delta 13C(ER) from Keeling plot analyses. The seasonal variation in delta 13C(ER) followed expected trends: during the summer dry season delta 13C(ER) became less negative (more enriched in 13C), but once rain returned in the fall, delta 13C(ER) decreased. However, we found no correlation between recent weather (e.g., vapor pressure deficit) and delta 13C(ER) either concurrently or with up to a one-week lag. Preliminary estimates suggest that the nocturnal CO2 flux advecting past the 28-m tower is a rather small fraction (<20%) of the watershed-scale respiration. This study demonstrates that monitoring the isotopic composition and CO2 concentration of cold air drainage at the base of a watershed provides a new tool for quantifying ecosystem metabolism in mountainous ecosystems on the basin scale.

A laboratory comparison of two methods used to estimate the isotopic composition of soil δ13CO2 efflux at steady state

The stable isotopic composition of soil (13)CO(2) flux is important for monitoring soil biological and physical processes. While several methods exist to measure the isotopic composition of soil flux, we do not know how effective each method is at achieving this goal. To provide clear evidence of the accuracy of current measurement techniques we created a column filled with quartz sand through which a gas of known isotopic composition (-34.2 per thousand) and concentration (3,000 ppm) diffused for 7 h. We used a static chamber at equilibrium and a soil probe technique to test whether they could identify the isotopic signature of the known gas source. The static chamber is designed to identify the source gas isotopic composition when in equilibrium with the soil gas, and the soil probe method relies on a mixing model of samples withdrawn from three gas wells at different depths to identify the gas source. We sampled from ports installed along the side of the sand column to describe the isotopic and concentration gradient as well as to serve as a control for the soil probe. The soil probe produced similar isotopic and concentration values as the control ports, as well as Keeling intercepts. The static chamber at equilibrium did not identify the source gas but, when applied in a two end-member mixing model, did produce a similar Keeling intercept produced from the control ports. Neither of the methods was able to identify the source gas via the Keeling plot method probably because CO(2) profiles did not reach isotopic steady state. Our results showed that the static chamber at equilibrium should be used only with a Keeling plot approach and that the soil probe is able to provide estimates of uncertainty for the isotopic composition of soil gas as well as information pertinent to the soil profile.