Alex P Lecture 11 Nutrients follows deep ocean circulation > 'Give info on source, flow dynamics, age etc. But nutrients not conservative > are added along flow path > so difficult to reconstruct into past. CO2 is exchanged between sea surface and air. Biological pump in surface ocean converts CO2 into organic matter via photosynthesis and biomineralization. > Links 813C to P via Redfield Ratio (C: N:P - 106:16:1) This sinks into deep ocean and is oxidised through respiration > most carbon released back into ocean. Some buried in sediments in deep ocean. Nutrient Based Proxies: 1. Carbon Isotopes Can use 813C (ratio of 13C to 12C) of calcite in foram shells. Note 813C standard reference is the Pee Dee Belemnite 813C is characteristic for different carbon environments: fossil fuels freshwater ECO2 biogenic methane + -50 marine org C land plants Atmospheric CO2 limestones soil org C soil CO 2 shallow ocean 2002 volcanic CO2 a deep ocean ECO2 40 -30 -20 -10 0 10 8130 % [after Trumbore and Druffel, 1995] Variability is due to 13C/12C fractionation (linked to rubisco enzyme) when DIC converted to organic carbon in surface ocean. 12C preferentially taken up in organic matter > gives low 813C in organic matter and high 813C in surface ocean. Deep ocean has low 813C as organic carbon released via respiration at depth.
Nutrients such as phosphate also taken up in surface ocean and released at depth - so have opposite profile: 813C, O/oo 0 1 2 O 1 2 0 2 13C PO4 DEPTH (km) 4- TTTTTT BOTTOM 777777 IS = 5 0 2 3 PO4 (umol/kg) Nutrients and organic matter added to water along flow path > Pacific and Indian Oceans have higher nutrients (NO3, Si, PO4~) and lower 813C and lower 02 (used up for respiration) than Atlantic. High 813C around Antarctic as deep water formation retains surface water signature. Ages and lowers 813C with flow. Can use depth transects of sediments to intersect major water masses > forams record 813C. Species: C. Wuellerstorfi. Used as lives at sedimentfiwater interface. Infaunal species would record pore water rather than sea water compositions. Carbon isotopes consisted of H2CO3 + HCO3fi + CO3fi (but due to pH are heavily weighted to HCO3fi). 13C is not temperature dependent > so 813C of CaCO3 reflects the 813C of total DIC in the ocean > ie just water composition. In Glacial Periods deep waters have lower 813C > due to restructuring of global ocean circulation (ie expansion of AABW, shallower NADW, sharper chemocline)
Cd/Ca Ratio Cd behaves like a nutrient (even though is biologically toxic) so has similar behaviour to PO4 -> both increase along deep water flow (Cd conc high in AABW) Cd is incorporated into calcite as an impurity (replaces Ca). So Cd/Ca in calcite gives Cd/Ca of seawater reconstruction > can then be linked to past PO4 and 813C seawater composition. Confirms that during glacial periods deep waters are more nutrient rich and more extensive, surface waters are depleted