Theodore J. Bohn & Dennis P. Lettenmaier

Civil and Environmental Engineering, University of Washington

Methane Emissions from Western Siberian Wetlands: Heterogeneity and Sensitivity to Climate Change

Wetlands play an important dual role in the global carbon cycle as both the largest natural methane source (115 TgCH4/y) and a large net carbon sink (76 TgC/y for high-latitude peatlands alone).  Both the extent of wetlands and the balance between their methane emissions and carbon sequestration depend on climatological and hydrological factors, leading to potentially significant feedbacks to the global climate system. This is especially true in northern Eurasia, where roughly 30 percent of global wetlands reside, and where ongoing and projected climate change is most pronounced.  Despite the importance of these systems to the global carbon cycle, large uncertainties remain in estimates of their extents, carbon fluxes, and responses to climate change.  To assess these uncertainties, we have coupled a large-scale hydrological model (VIC) to a terrestrial carbon model (BETHY) and the wetland methane emissions model of Walter and Heimann, and applied this modeling framework to the Chaya basin in the western Siberian lowlands.  Here we present the results of an analysis of the sensitivity of wetland methane emissions to temperature, precipitation, and water table depth for the period 1980-1999.

Carlos Sierra1, Henry W. Loescher1, Flavio Moreno2, Jorge del Valle2, Sergio Orrego2, & Mark Harmon1

1Forest Science, Oregon State University, 2Departamento de Ciencias Forestales, Universidad Nacional de Colombia Sede Medellin

Assessing the Role of Tropical Forests in the Global Carbon Cycle: The Role of Spatial Heterogeneity on Temporal Variability and Error

It has been hypothesized that mature tropical forests are responding to current temperatures and atmospheric concentrations of CO2 with a significant increase in biomass production and net carbon accumulation. However, little or incomplete data is available to test this hypothesis. We developed a very detailed carbon balance for a mature tropical forest in Colombia. We estimated total carbon stocks, net primary production, heterotrophic respiration, and net ecosystem production (NEP) for two consecutive intervals. Spatial and temporal variability of these variables was estimated using Monte Carlo methods to obtain 95% confidence intervals of carbon stocks and fluxes for this region. We used an ecosystem carbon model (STANDCARB) to predict the degree of variation of carbon fluxes due to gap dynamics and climate for this forest. We found that the net carbon flux for this forest, measured as NEP was between -4.03 and 2.22 Mg C ha-1 yr-1 for the period 2000-2002. This range of variation was not statistically different than the range of variation predicted from the ecosystem model as an effect of gap dynamics. We found that previous studies of Net Ecosystem Exchange in the tropics were also within our observed and hypothetical range. Only two studies have shown large deviations from these ranges and were highly associated with La Niña events. However, this response was not consistent in other forests, including our sites, suggesting that the effects of climate change may not be homogenous over the tropics. A modeling exercise suggested that gap dynamics may play a critical role in the response of tropical ecosystems to climatic perturbations. Our current research suggests that variability and uncertainty in current measurements may not adequately capture the degree at which climate change is affecting forest ecosystems.

David Munro

Oceanography, University of Washington

Application of Oxygen Isotopes to Carbon Cycling off the Coast of Southern California

Despite the fundamental importance of primary production (PP) to marine ecosystems and carbon cycle dynamics, large-scale PP estimates remain poorly constrained, particularly for dynamic coastal regions. Remote sensing provides an expansive view of ocean processes on a daily basis, but converting satellite data to quantitative PP estimates requires algorithms that, to date, have relied on in vitro 14C-based PP measurements. A relatively new in situ technique, the oxygen isotope method (OIM), provides expanded spatiotemporal resolution and avoids in vitro effects associated with 14C-derived PP. Contemporaneous measurement of O2/Ar gas ratio provides an estimate of the biological oxygen saturation state. Together, these measurements yield the ratio of net community production (NCP) to gross primary production (GPP), a useful indicator of carbon export efficiency. I measured the oxygen triple isotopic composition of dissolved O2 and the dissolved O2/Ar gas ratio of seawater samples on five CalCOFI cruises between November 2005 and January 2007. Additionally, samples were collected from the seawater intake system of the RV Thompson during a transit cruise from Seattle to San Diego during March 2007. The objective of these measurements was to determine mixed layer GPP and N/G and to compare in situ rates to 14C PP rates routinely measured on CalCOFI cruises.

Lia Slemons

Oceanography, University of Washington

Journey Across the Pacific’s Undersea River: Trace Metal Distributions of Iron, Aluminum, and Manganese in the Equatorial Undercurrent

Upwelling by the cold, nutrient-rich waters of the Equatorial Undercurrent (EUC) in the eastern Pacific maintains carbon flux to the atmosphere on the order of 0.9 Pg C/yr and high-nitrate, low-chlorophyll conditions.  Biological productivity in this region is regulated by grazing pressure and iron limitation, but iron sources are poorly constrained.  Transport of subsurface iron in the EUC from western terrestrial sources has been suggested based on profiles collected at 140°W, but field samples are limited.  Up until now, there have been no zonal sections of iron or other trace metals in the western equatorial Pacific.  We measured iron, manganese, and aluminum along the Equator and at 2° N and 2° S from 140°W to 145°E to a depth of 1000 m.  The depth range of the EUC was sampled with the greatest vertical resolution.  Southern EUC source waters were also measured.  Aluminum shows a strong maximum at the density of the EUC core which increases westward towards the candidate source.  The subsurface maximum of iron is also greatest in the west, but the zonal gradient is less consistent and the maximum deeper than that of aluminum.  Manganese is strongly surface-enriched west of the dateline.  The varying distributions of these three trace metals suggests differences in biogeochemical cycling.  The transport and loss of iron in the equatorial Pacific influences the magnitude of biological production and thus carbon export in the eastern Pacific, one of the most productive regions of the open ocean.