Intoduction to Marine Geology and Geophysics 11/1 Mid Term Sediments, Processes, and the Sedimentary Record 11/6 (McManus) Deep-sea sediments: composition, distribution 11/8 (McManus) Biological, chemical, and physical abyssal processes 11/13 (McManus)Dating methods and the sedimentary record 11/15 (McManus)Paleo-environmental proxies 11/20(McManus)Deep water chemistry and atmospheric p(CO 2 ) 11/22 Thanksgiving 11/27 (Hoffmann)Paleothermometry 11/29 (Thompson)Pleistocene ice-age cycles 12/4 (McManus)Sedimentary records of abrupt climate change 12/6 Final Exam

black body A theoretical object that absorbs all incoming electromagnetic radiation. It emits radiation as a function of temperature.

Atmospheric CO 2 I Greenhouse gases and the temperature of the Earth II Ice core evidence that glacial pCO 2 was 80 ppm lower. III Could it be due to terrestrial biosphere change? No! IV How did the ocean do it? Physical - chemical property changes (T, S) Physical pumps Biological pumps (nutrients, C org, alkalinity)

Blackbody temperature The sun radiates primarily in the visible. Earth radiates in the IR. Earth has a blackbody Temperature below the freezing point of water. Sun (T~ 5780°K) Earth (T~ 255°K) Wavelength (microns)

Greenhouse effect Atmosphere allows visible light to pass, but ‘greenhouse’ gases (H 2 0, CO 2, CH 4 ) trap outgoing infrared and warm the Earth.

Vostok ice core (Antarctica) CO 2 and  D are nearly in phase. Both lead  18 O atm O 2 (ice volume). Both lead Greenland  D and  18 O ice (Northern hemisphere temperature). Phase relationships (relative timing) These allow us to rule out several possible mechanisms for driving the CO 2 changes.

Siegenthaler et al. (2005) CO 2 (ppmv)  D (‰) Temperature Epica Dome C ice core (Antarctica) CO 2 (greenhouse gas) and  D (temperature) vary repeatedly through multiple glacial cycles. The variations are approximately 90 ppmv.

Vostok ice core (Antarctica) CO 2 and  D are nearly in phase. Both lead  18 O atm O 2 (ice volume). Both lead Greenland  D and  18 O ice (Northern hemisphere temperature). Time

Carbon reservoirs Most carbon is in solid earth. Ocean has most of the rest (60:1) Atmosphere small reservoir, but important!

Isotopic fractionation Stable isotope fractionation may be diagnostic tool.

Carbon reservoirs Different carbon pools are isotopically distinct.

Ocean carbon shift Mean ocean isotopic ratio changed during ice age.

Climate and Land Climate changes themselves cannot account for the observed changes in CO 2. Changes in the terrestrial biosphere, evident in carbon isotopes in the ocean, are too small to explain CO 2 change, and they indicate a shift in the wrong direction!

Biological pumps Productivity near the sea surface changes the chemistry of both the surface and deep ocean.

CO 2 in the ocean The inorganic as well as organic chemistry of CO 2 in the ocean plays an important role.

CO 2 cycling Organic carbon is produced by photosynthesis and then recycled through respiration in the deep ocean. Both processes influence the total dissolved carbon and the dissolved carbonate ion concentration, but the effect may not be intuitive.

Alkalinity Excess positive charge of dissolved ions to be balanced. In the case of carbonate alkalinity, the net positive charge is balanced by a combination of [CO 3 = ] and [HCO 3 _ ].

Coral reef hypothesis Growth of coral reefs on flooded margins as sea level rises would have the effect of increasing atmospheric pCO 2, but the timing is wrong.

Phosphate burial hypothesis Burial of phosphate and other nutrients on shelves as sea level falls, and then release during sea level rise would have the right effect on atmospheric pCO 2, but again, the timing is wrong.

Influence of pumps Various pumps would have differing effects on atmospheric pCO 2, and also on the isotopic composition of dissolved inorganic carbon (DIC).

Productivity Biological activity results in systematic changes in concentration and isotopic ratio of bio-limiting and bio-intermediate elements.

Carbon isotopes Photosynthetic fractionation of organic carbon leaves seawater enriched in heavier carbon-13. The resulting Isotopic ratio in seawater is then incorporated in CaCO 3, providing a nutrient-tracer.

Isotopic influences Photosynthetic fractionation results in a strong negative correlation between nutrients and carbon isotopes. Gas exchange, local productivity, and global reservoir shifts can also influence  13 C. So  13 C can be used as a tracer for water masses (circulation), although gradients are more reliable than the absolute values.

NADW GEOSECS The meridional overturning circulation (MOC) produces North Atlantic Deep Water (NADW). Evident in salinity and many other properties…

NADW Kroopnick (1985) Also evident in carbon isotopes (  13 C). The meridional overturning circulation (MOC) produces North Atlantic Deep Water (NADW).

LGM meridional section, western basin Curry and Oppo (2005) Paleocean circulation The configuration was different, but not the rate of circulation?

Cadmium as a tracer Dissolved cadmium is strongly correlated with phosphate and nitrate. Cd a “nutrient” tracer High Cd = high nutrients

Cadmium as a tracer Dissolved cadmium in bottom water is reflected in benthic foraminifera shells.

Ocean circulation ‘Biological pump’ and ‘conveyor belt’ combine to distribute nutrients and ‘nutrient proxies’ in ocean.

Combined proxies Both carbon isotope and cadmium tracers support repeated glacial to interglacial changes in ocean circulation combine to distribute nutrients and ‘nutrient proxies’ in ocean.

Nutrient changes and pCO 2. Changes in the inventory or whole ocean distribution of nutrients could explain the observed shifts in pCO 2. But there is no evidence for change in whole ocean inventory, and no evidence for widespread oxygen depletion in the deep ocean. Signal is most evident in Atlantic, and is most likely circulation.

Nutrient shift Carbon isotopes suggest a wholesale shift to lower values. Cadmium harder to discern. If anything, there is a small change to lower values in glacial, opposite required shift.

Ocean carbon shift Mean ocean isotopic ratio changed during ice age.

Ocean carbon shift Most likely caused by decrease in terrestrial biosphere. Too small and with the wrong sense to drive changes in pCO 2, but enough to change ocean reservoir.

Carbon shift hypothesis A change in the ratio of inorganic to organic carbon produced in the surface ocean and exported to depth might help explain the changes in atmospheric in pCO 2.

Modeled carbon shift A geochemical modeling experiment failed to show that the change in carbon ratio could shift the lysocline enough to have the required change in atmospheric pCO 2.

Focus on Southern Ocean It is the main region where deep ocean and in atmosphere are in nearly direct contact.

Southern Ocean Several models reveal strong connection to pCO 2.

Southern Ocean Nutrient utilization (dust?), stratification (sea ice?), and/or circulation combined with carbonate compensation Remain the leading explanations for CO 2 change.

Southern Ocean Nutrient and sea ice connection to pCO 2.

Southern Ocean Nutrient, dust, and sea ice connection to pCO 2. Remains the leading explanation.

Isotopic influences Photosynthesis and temperature equilibration are competing influences on seawater  13 C.