Final Report Summary - CARBONSINK (Life beneath the ocean floor: The subsurface sink of carbon in the marine environment)
The primary objective of this proposal was to better understand what happens to carbon deposited in the bottom of the ocean. This removal of carbon from the surface of the planet is a critical component of the long-term carbon cycle. However, what is deposited at the bottom of the ocean is not representative of what is ultimately buried within sediments are populations of bacteria and archaea living largely in the absence of oxygen. These microbial populations consume organic carbon that has fallen to the sea floor, producing a large excess of dissolved inorganic carbon. Before this project we didn’t know what happened to this dissolved inorganic carbon; did it diffuse back into the ocean or was it precipitated as other minerals in the sediment? What controlled its fate, and what role did this play in the carbon cycle?
In this project I developed a new technique using calcium isotopes to help resolve where dissolved inorganic carbon gets precipitated in mineral form, as well as calculated for the first time how much gets tied up in mineral form, and how much returns to the ocean. We were able to show that, today, most of the dissolved inorganic carbon returns to the ocean and only a small amount is precipitated as minerals in the sediment. The fraction that precipitates comprises 3% of the total removal of carbon as carbonate. However, in the geological past this was likely much higher, as we were able to show that the amount that is removed as minerals versus returned to the ocean and the effect this has on the carbon cycle strongly depends on the amount of methane in sediments. When methane is present, the precipitation rate of minerals from dissolved inorganic carbon in sediments is 25x higher than when methane isn’t present. We were also able to show that today, the return of the dissolved inorganic carbon to the ocean influences the chemistry of the deep ocean. A secondary result from the project was we were able to use calcium isotopes to understand where and how carbonate minerals are precipitating in the oceanic crust.
In this project I developed a new technique using calcium isotopes to help resolve where dissolved inorganic carbon gets precipitated in mineral form, as well as calculated for the first time how much gets tied up in mineral form, and how much returns to the ocean. We were able to show that, today, most of the dissolved inorganic carbon returns to the ocean and only a small amount is precipitated as minerals in the sediment. The fraction that precipitates comprises 3% of the total removal of carbon as carbonate. However, in the geological past this was likely much higher, as we were able to show that the amount that is removed as minerals versus returned to the ocean and the effect this has on the carbon cycle strongly depends on the amount of methane in sediments. When methane is present, the precipitation rate of minerals from dissolved inorganic carbon in sediments is 25x higher than when methane isn’t present. We were also able to show that today, the return of the dissolved inorganic carbon to the ocean influences the chemistry of the deep ocean. A secondary result from the project was we were able to use calcium isotopes to understand where and how carbonate minerals are precipitating in the oceanic crust.