In order to improve our understanding of natural controls on atmospheric CO2, we are investigating why atmospheric CO2 varies with recent climate cycles, being lower during ice ages. Work conducted previously by CMI members and others suggests that the Southern Ocean, the continuous band of ocean surrounding the Antarctic continent, holds the answer.
Southern Ocean nutrients and glacial cycles
One hypothesis regarding glacial-interglacial cycles is that large scale iron fertilization by increased dust deposition stimulated biological productivity in the Southern Ocean, leading to increased oceanic uptake of atmospheric CO2. Therefore, the interactions between iron and biological productivity in the Southern Ocean are closely linked to paleoclimate. Bender’s group has synthesized a large dataset of net community production and gross primary production based on shipboard observations in the Southern Ocean and integrated the results with values of iron deposition rates simulated by S.-M. Fan at GFDL. This dataset is unique because it reflects production over the timescale of order one week, which is also the fundamental response time of oceanic plankton to environmental changes. Analysis of these data suggests a significant link between transient iron inputs and net community production, which supports the idea that addition of iron to these ecosystems stimulates a bloom.
Sarmiento’s group is exploring how mechanisms of variability in ocean circulation impact the uptake of atmospheric CO2 on centennial and millennial timescales, as reflected by marine sediment records. In order to apply the GFDL coupled ocean/ecosystem model to these longtimescale problems, a fast version of the model was developed during 2006. This model configuration requires only a fraction of the computational cost of the standard GFDL model, while still using the same biogeochemical model. Preliminary results show a previously unrecognized control on nutrient availability to the ocean surface of the southern hemisphere by the position of southern westerly winds, with important implications for CO2. Sarmiento’s group has also successfully carried out long-term iron fertilization experiments in order to test the hypothesis that higher glacial dust fluxes to the ocean caused greater CO2 storage by increasing phytoplankton growth rates. This work builds upon previous CMI-funded work in Sigman’s group, which indicated more complete consumption of nutrients in the Subantarctic Ocean during the last ice age.
Polar stratification during glacial periods
Daniel Sigman and his collaborators continue to pursue the evidence for reduced vertical exchange (i.e. “stratification”) in the halocline-bearing polar ocean regions under colder climates of the past 3 million years. A major motivation for this focus is that the reconstructed polar ocean changes have the capacity to explain the low atmospheric concentration of CO2 during ice ages, with polar stratification storing CO2 in the abyssal ocean. Recent analysis of N isotopic data by Sigman’s group indicates reduced nutrient supply to the Bering Sea surface from below during the last ice age, strengthening the case for a bipolar (Antarctic and North Pacific) increase in stratification during ice ages. This finding constrains the cause for polar ocean stratification upon cooling to a mechanism that applies to both of these regions.
Model simulations carried out by AOS postdoc Agatha de Boer showed that the reduced sensitivity of seawater density to temperature at low temperatures provides such a mechanism. In the case of a globally colder ocean, the drive toward Antarctic and North Pacific overturning due to temperature becomes weaker, allowing the net atmospheric deposition of fresh water on these polar ocean regions to stratify them. de Boer has demonstrated that the density/temperature effect described above fits into a common theoretical framework with the ocean’s response to changes in southern hemisphere winds, thus relating to the ongoing work on wind/CO2 connections mentioned above.
Impacts of recent climate variability on human societies
Daniel Sigman’s long term collaboration with Gerald Haug of GFZ Potsdam has yielded a reconstruction of East Asian winter monsoon strength over the last 16 thousand years, based on sediment cores from Lake Huguang Maar in southeastern China. This study indicates a strong anti-correlation with the summer monsoon, and connections with our previous work in Cariaco Basin off Venezuela indicate Pan-Pacific shifts in tropical climate over the past 16 thousand years that may be explained by migration of the intertropical convergence zone. The coincident timings of multi-annual dry periods in both East Asia and Central America between AD 700 and 900 and their temporal correspondence with human societal events in both regions raise the possibility that Pacific-wide migrations in the tropical rain belt contributed to the coincident declines of both the Tang dynasty in China and the Classic Maya in Central America.