Department of Earth and Environmental Science

Irina Marinov

Research

Response of Ocean Ecology to future climate change
(Irina Marinov with Scott Doney and Ivan Lima (WHOI), Keith Lindsey (NCAR) and Keith Moore (UCSC))
How will ocean ecology change in the next one hundred years of increasing atmospheric CO2 concentrations? In this project we analyze the impact of increasing CO2 on phytoplankton distribution, abundance and competition in the most recent IPCC type simulations performed with the NCAR CCSM ocean-atmosphere-land coupled model. We are presently studying how different ecological biomes expand or contract with increasing CO2 and the link between changes in ocean physics, changes in phytoplankton distribution, surface nutrients, and the carbon cycle.

How the ocean carbon pumps control atmospheric pCO2. The impact of future changes in ocean ventilation on ocean carbon pumps and atmospheric pCO2.
Razvan Zarzu and Irina Marinov, with Jaime Palter (Princeton)

We plan to develop an expanded and improved theoretical understanding of ocean carbon pumps. This will be achieved by including the effects of the carbonate, solubility pumps and anthropogenic emissions in our previous simplified theoretical framework (Marinov et al. 2008b). The ventilation of the deep and intermediate layers of the ocean is accomplished by a complex suite of processes, each of which will be affected by a warming climate. Because ocean ventilation provides a pathway for atmospheric carbon dioxide—both natural and anthropogenic—to invade a massive volume of the ocean, changes in ventilation are likely to have profound implications for global climate. Our theory will be used to predict the potential impact of changes in ventilation on ocean natural carbon pumps and anthropogenic carbon uptake and the resulting feedbacks on atmospheric pCO2.

Biological-physical controls on the large scale air-sea CO2 flux distributions. The compensation mechanism.
(Irina Marinov with Anand Gnanadesikan of NOAA/GFDL)

This work explores the impact of circulation changes on the meridional distribution of the steady state air-sea CO2 fluxes. Changes in circulation resulting from modifications in diapycnal mixing or changes in Southern Ocean winds affect both remineralized phosphate and temperature. In the Princeton GCM the biological CO2 flux, which is a function of remineralized phosphate, and the solubility CO2 flux, which is a function of temperature or heat transport, change with diapycnal mixing and Southern Ocean wind magnitude in opposite ways. Thus, while both the biological and solubility air-sea CO2 fluxes vary strongly with diapycnal mixing or wind magnitude, the full (solubility+biological) air-sea CO2 flux shows nearly no variation. If our result holds in the real ocean, it could potentially imply that large changes in the air-sea carbon distribution (due, for example, to the observed increase in Southern Ocean winds) would not be reflected in the observed air-sea CO2 flux. Since surface restoring of temperature and salinity constraints strongly the solubility air-sea flux, while surface restoring of nutrients constraint the biological air-sea flux, this result highlights the need for using realistic boundary conditions in climate simulations. The validity of this result needs to be tested in more complex models in which surface boundary conditions are allowed to vary in response to circulation changes.

Previous Research Projects:

The Southern Ocean Biogeochemical Divide
(I. Marinov, A. Gnanadesikan, R. Toggweiler and J.L. Sarmiento, published in Nature, June 2006)

Previous studies have shown that the Southern Ocean is crucial in controlling the atmosphere-ocean balance of carbon dioxide as well as global biological production. Here we demonstrate that two separate regions of the Southern Ocean (the Antarctic and the Subantarctic) control the air-sea carbon dioxide balance and global biological production. The biogeochemical divide between these two regions is likely located in the vicinity of the Polar Front. The large-scale pattern of circulation in the Southern Ocean involves upwelling of deep water, some of which flows to the south to sink as bottom water, and some of which flows to the north to form intermediate and mode waters. We show that the air-sea balance of carbon dioxide is controlled primarily by the biological pump and circulation in the deep-water formation region, whereas global biological productivity is controlled primarily by the biological pump and circulation in the intermediate and mode water formation region. This implies that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.

Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO2
(Marinov, I., A. Gnanadesikan, J.L. Sarmiento, J.R. Toggweiler, M. Follows and B. Mignone, published in GBC, 2008)

The atmospheric carbon dioxide partial pressure (pCO2) is set to a large degree by the biological storage of carbon in the deep ocean. A more efficient biological pump results in more carbon storage in the deep ocean and smaller atmospheric pCO2. This study shows that diapycnal mixing, isopycnal mixing and Southern Ocean winds, by changing the Southern Ocean overturning circulation, influence strongly the biological storage of carbon in the ocean and atmospheric pCO2 in a realistic ocean General Circulation Model (GCM). Increased diapycnal mixing and Southern Ocean winds result in less ocean carbon storage and higher atmospheric pCO2, in agreement with earlier box model studies. By contrast, increased isopycnal mixing is shown to increase the storage of carbon in the ocean and decrease atmospheric pCO2.
Additionally, this paper attempts to clarify a longstanding confusion in the oceanic community about what controls the biological storage of carbon in the ocean. As such, we show conclusively that surface nutrients and biological export production are not good metrics for ocean carbon storage and atmospheric pCO2. By contrast, we show that the fraction of nutrients in the global remineralized pool or the fraction of nutrients in the global preformed pool are excellent indicators for atmospheric pCO2. We develop a simple theory relating global preformed nutrient concentrations and atmospheric pCO2.

How does atmospheric pCO2 respond to changes in surface nutrients, such as those associated with iron fertilization of the surface ocean?

Atmospheric pCO2 sensitivity to nutrient depletion in the ocean: theory and models by I. Marinov, M. Follows, A. Gnanadesikan, J.L. Sarmiento and R. Slater, published in JGR Oceans, 2008.

Iron fertilization of the HNLC (high-nutrient low-chlorophyll) ocean areas has been proposed as a mechanism to decrease atmospheric CO2 levels through the associated increase in biological production. Increased surface biological production through iron fertilization is one of the leading hypotheses for explaining the lower atmospheric pCO2 observed during glacial times [Martin, 1990]. In the present work we study the impact of increasing biological production on atmospheric pCO2 in the Princeton GCM. The method used is to deplete (i.e., force towards zero) nutrients in large ocean areas and convert them to export production. Here we show how the uptake of atmospheric CO2 following nutrient depletion depends on the region depleted, gas exchange rate, ice, and oceanic circulation. We show that the Southern Ocean (and in particular the Antarctic region south of the Polar Front) is the most important area for CO2 uptake in all models studied. We show that the outcome of nutrient depletion experiments depends critically on the circulation of the ocean, which changes as diapycnal mixing and Southern Ocean winds change. Depleting surface nutrients changes both deep preformed nutrients and the CO2 disequilibrium at the ocean surface, with implications for the total carbon storage in the ocean. We examine how these two effects contribute to changes in atmospheric pCO2 in the context of models with different circulations. In conclusion, we show that ocean physics and the details of the air-sea gas exchange mechanism are crucial in determining the atmospheric pCO2 response to surface nutrient depletion.

Export is not enough: Nutrient cycling and carbon sequestration
Anand Gnanadesikan and Irina Marinov
published in 2008 in the Marine Ecology Progress Series Theme Section (MEPS-TS) "Implications of large scale iron fertilization of the oceans”

The question of whether iron fertilization can yield verifiable carbon sequestration is often cast in terms of whether fertilization results in enhanced particle export. However, models studies show that oceanic carbon storage is only weakly related to global particle export - depending instead on an increase in the carbon associated with the global pool of remineralized nutrients. As a result, local balances are unlikely to describe the global impact of fertilization. Effects that are remote from the fertilization site in time or space, such as reduction in productivity, changes in stoichiometric ratios or changes in the disequilibrium of sinking water can significantly affect the impact of fertilization on atmospheric carbon dioxide. Understanding these effects and constructing models, which accurately represent them, is thus a crucial part of designing large-scale fertilization projects.

Other projects of interest:

Impacts of increasing Southern Ocean winds on oceanic convective processes, Southern Ocean ecology and the global carbon cycle

Coupled model simulations show clearly that a major source of uncertainty in IPCC type predictions of future climate is the wide range of predictions of water mass transformations in the Southern Ocean and shelf areas around Antarctica (Friedlingstein et al., 2006, Russell et al., 2006).

Southern Hemisphere westerly winds play a crucial role in setting up the stratification and large scale circulation in the Southern Ocean, influencing the amount of upwelling, downwelling and lateral mixing in the region [Toggweiler and Samuels, 1993]. Recent research shows that in the past 40 years Southern Hemisphere westerlies shifted poleward and the zonal wind speeds increased [Thomson and Solomon, 2002, Kalnay et al., 1996]. The obvious question to ask is: What is the impact of changing winds on the oceanic solubility and biological “natural” carbon pumps, the net anthropogenic carbon uptake as well as on regional ecology?

Interactions of Southern Ocean dynamics with local biogeochemistry are critical for the global CO2 sink and other tracer distributions. Our research to date suggests that deep water formation areas in the Southern Ocean are mostly responsible for setting the air-sea carbon balance on long time scales (of thousands of years). Most of the deep water formation in General Circulation Models (GCMs) occurs in deep water formation centers such as the Weddell Sea. While Weddell Sea polynias have been noticed in nature, observational work seems to suggest that most deep water formation in the Southern Ocean occurs on shelves. It is unclear how accounting for more realistic deep water formation processes in GCMs would impact the carbon cycle and climate changes. The broad questions I am interested in are: What is the role of convection in the exchange of carbon between the atmosphere and the deep ocean? What percent of Southern Ocean convection occurs on shelves and what percent occurs in the open ocean? How does deep ocean convection work and what are the impacts of convection on surface nutrient uptake, local ecology, air-sea CO2 and O2 exchange and ultimately on carbon storage in the ocean?

Understanding the mechanisms behind glacial-interglacial climate changes: the role of oceanic stratification

It is likely that a combination of changes in ocean stratification, nutrient uptake and ice coverage are responsible for the observed glacial-interglacial differences in atmospheric CO2. The exact contribution of each of these changes to the observed 80 ppm drop in atmospheric carbon dioxide during glacial periods is still unknown. The questions I would like to pursue are: How has the large-scale circulation and stratification changed in the past and how do we expect it to change in the future? How has the intensity and spread of deep water formation changed on glacial-interglacial scales and how is it likely to change with future climate change? What are the impacts of these changes in circulation on atmospheric pCO2 and ocean biogeochemistry?

I am puzzled by the disagreement between the climate modeling community and paleoclimatologists regarding the connection between Southern Ocean stratification and atmospheric pCO2. While paleoclimate data suggests decreased oceanic stratification and more deep water formation during warm climates [e.g., Francois et al., 1997, Sigman and Boyle, 2000], climate modeling studies predict an increase in water column stratification and a decrease in high latitude convective overturning with warming [e.g., Sarmiento et al., 1998, Stouffer and Manabe, 1999]. The inadequate representation of ice dynamics, Southern Ocean westerly position relative to the Drake Passage [Russell et al., 2006] and Southern Ocean deep water formation in global models might help explain the discrepancy between models and observations. This is a fascinating issue for further exploration in both models and observational work.