CO 2 is the raw material used to build biomass (reduced to form organic matter)

Transcription

1 1. The oceanic carbon system (a) Significance (b) CO 2 speciation (c) Total CO 2 (d) Atmosphere-ocean CO 2 exchange (e) Global status of CO 2 2. Human perturbations of N and P cycling 3. Other elements: (a) Silicon (b) Iron (c) Passive uptake (d) Trace element abundances 4. Summary 1

2 CO 2 is the raw material used to build biomass (reduced to form organic matter) CO 2 controls the fraction of inbound radiation that remains trapped in the atmosphere (greenhouse effect), which controls planetary climate CO 2 controls the acidity (ph) of the oceans Distribution of CO 2 species affects the preservation of CaCO 3 deposited on the sea floor CO 2 (g) reacts extensively upon contact with H 2 O Major dissolved forms: CO 2 (aq) (aqueous carbon dioxide) HCO 3 - (bicarbonate ion) CO 3 2- (carbonate ion) Species interconvert readily Changes to one part of CO 2 system lead to redistribution of all CO 2 species Reactions not always intuitive 2

3 Speciation equations The equilibrium of gaseous and aqueous carbon dioxide: CO 2(g) CO 2(aq) Subsequent hydration and dissociation reactions: CO 2(aq) + H 2 O HCO H + HCO 3 - CO H + CO 2 (g) CO 2 (aq) + H 2 O H 2 CO 3 (aq) H + + HCO 3- (aq) H+ + CO 3 2- (aq) WGBU (2006) from concepts in Raven et al. (2005) 3

4 Total CO 2 = [CO 2(aq) ] + [H 2 CO 3 - ] + [HCO 3- ] + [CO 3-2 ] At seawater ph, > 99 % of CO 2 species are HCO 3 - and CO 3 2-, so we can simplify: Total CO 2 = [HCO 3- ] + [CO 3-2 ] Total CO 2 = ΣCO 2 = Total Inorganic Carbon = Dissolved Inorganic Carbon (DIC) Data from HOT-DOGS (see previous lecture) Libes (1992) 4

5 10/25/12 In the atmosphere (ptotal = 1 atm), the individual gases have separate partial pressures, which is what is reported: po2 = 0.21 atm pn2 = 0.78 atm pco2 = atm CO2 exchanges with seawater via Henry's Law: [CO2]sw = k pco2 When air and sea are at equilibrium, pco2sw = pco2atm Transfer rate increases with higher wind speed and surface turbulence Gases are more soluble in cold water Deep water, during its formation, equilibrates with current atmospheric CO2 levels NADW net CO2 flux = 0.26 x 1015 g C y-1 PMEL/NOAA, 5

6 10/25/12 PMEL/NOAA, Libes (1992) Positive values at equator (esp. in the Pacific) and along west coasts are from upwelling and subsequent gas evasion to atmosphere Negative values where there is high bioproductivity, and where cooling of ocean increases solubility of gas and causes gas transfer into surface ocean 6

7 10/25/12 Dark line - Atmospheric pco2 record (solid line) at Mauna Loa Dots and violet lines - Monthly surface water pco2 in north subtropical gyres Left: Pacific (Hawaii Ocean Time-series) Right: Atlantic (Bermuda-Atlantic Time Series) Kleypas et al. (2006) Seasonal oscillations are detectable Oceanic pco2 is out of phase with atmospheric pco2, due to mixing of deep, high pco2 seawater during winter/spring Much higher variability in pco2 in the Atlantic, due to much greater annual changes in mixed-layer depth Surface water at BATS station is supersaturated with CO2 after spring mixing, and therefore is a source of CO2 to the atmosphere (compare with values shown in previous slide). Libes (1992) 7

8 There is about 50 times as much CO 2 in the ocean as in the atmosphere Primary production removes atmospheric CO 2 from the surface ocean to deep waters as organic carbon and CaCO 3 (this process is referred to as the biological pump) Most of the organic carbon is remineralized in the water column The sedimentary CaCO 3 sink is 4 times greater than the organic carbon sink Human CO 2 input to the atmosphere (6 x g C y -1 ) is primarily through burning of fossil organic matter (i.e., oil, coal and natural gas) and cement production The input is considerable, but up to half can be accounted for in the atmosphere (boreal forests are a significant sink) The input is detectable in ocean waters as: Rising pco 2 Decreasing ph Feely et al. (2009) 8

9 Biomineralization: the formation of mineral tissues by organisms CaCO 3 biominerals: Calcite: coccolithophorids, foraminifera, molluscs Aragonite: corals Ca 2+ + CO 3 2- CaCO 3 CaCO 3 biomineralization depends on the saturation state (Ω) of a biomineral in seawater: Ω Description Process < 1 Undersaturation Net dissolution 1 Saturation Equilibrium > 1 Supersaturation Net precipitation Since Ca is quite abundant and doesn t vary much in seawater, variations in CO 3 2- are more likely to control Ω Feely et al. (2009) 9

10 10/25/12 Human perturbations of N and P cycling Sewage and fertilizer use have increased N and P input to the oceans, mainly through riverine and surface discharges, but also through submarine groundwater discharge These inputs have increased NPP in coastal areas, and perhaps ocean-wide, by partly relieving any nutrient limitation Coastal ocean is the one most impacted: 60 % of the Earth s population lives within 60 km of the ocean Over 50 % of these live in cities Even though it accounts for 10 % of the ocean s surface, it accounts for 18 % of NPP and for 83 % of the C buried Greater NPP could result in greater export of POC to deep sea, potentially serving as a sink for increasing atmospheric CO2 However, based on the model calculations, the estimated increased C uptake ( x 1015 g C y-1) is small relative to the anthropogenic C release to atmosphere (6 x 1015 g C y-1) 10

11 Si is used for the biomineralization of opal by diatoms, a major group of photosynthesizers, as well as radiolarians, a widespread planktonic protozoan group Si is, therefore, considered a macronutrient Biotic uptake draws down dissolved Si in oceanic surface waters Biomineralized opal dissolves along with the organic matter of sinking particles Si thus displays a vertical profile similar to other macronutrients Data from HOT-DOGS (see previous lecture) Fe has now been recognized as an important limiting micronutrient for much of the ocean It was originally pursued as an explanation for highnutrient low-chlorophyll (HNLC) regions of the ocean Fe addition by dust deposition was postulated to drive productivity Successfully tested by Fe addition experiments, which led to increased productivity Has led to ideas for draw-down of atmospheric CO 2 (geoengineering) 11

12 Fe addition experiments Powell (2007) oceanus/viewarticle.do?id=34167 Armbrust (2009) Passive uptake is the uptake of biochemically non-essential elements by living organisms Passive uptake most likely happens by co-precipitation or adsorption Therefore, it is possible that the geochemistry of many trace elements in seawater is controlled directly or indirectly by the biota whether these elements are essential or not For example, Cd appears to substitute for Zn in certain biomolecules, and Cd is well correlated with P in seawater 12

13 Consequently, Cd displays a nutrient-like profile in seawater Cd:Ca in forams has been used as a paleo-p indicator The geochemistry of elements that are taken up actively or passively by biota is reflected in their mean residence times in seawater 13

14 The oceanic CO 2 system is dynamic, with several actively interacting pools on the earth s surface, the biggest of which is the ocean Atmospheric CO 2 equilibrates with the oceanic CO 2 pool, and this is now documented by oceanic time-series measurements Biomineralization of calcium carbonate acts as an important sink for carbon in the ocean Changes in atmospheric CO 2 are starting to have a profound impact on the chemistry and biology of the upper ocean Human introduction of N and P to the oceans may increase NPP and consequently C removal Silicon, as well as iron and other trace elements, display nutrientlike profiles with depth, either because they are essential for biota or because they are taken up passively by them 14