What caused Glacial-Interglacial CO2 Change? Douglas L. Love Meto 658A Spring 2006
Suggested papers: Reviews: Archer et al., 2000 Newer ideas: Zeng 2003 Toggweiler et al. 2005 Paillard and Perenin 2005 • Broecker and Henderson 1998 Broecker and Peng 1998
Archer et al, 2000 David Archer Arne Winguth David Lea Natalie Mahowald U Chicago U Wisconsin UCSB NCAR
Archer et al, 2005 • corellation between • pCO2 • Ice volume • Temperature records Glacial pCO2 was 80-90 μatm lower than interglacial Radiative forcing from CO2 accounts for half of climate change Tight repeatable
Archer et al, 2005 • Glacial pCO2 was 80-90 μatm lower than in the interglacial • Radiative forcing from CO2 accounts for • half of climate change • “The terrestrial biosphere and soil carbon reservoirs would have to be approximately double in size to deplete pCO2 by 80 μatm.” • “δ13C from deep-sea CaCO3, more 12C rich during glacial time, tells us that if anything, the terrestrial biosphere released carbon during glacial time [Shackleton, 1977]”
Archer et al, 2000 • Advances and retreats of ice sheets • Documented by isotopic composition of seawater • Oxygen in CaCO3: • 16O is selectively sequestered in glacial ice. • Oceans become enriched in 18O Glacial cycles:
Archer et al, 2005 • Clear physical link between Northern Hemisphere • summer heating and ice sheets • No easy link from orbital variations to pCO2. • pCO2 rise clearly precedes the 18O of the atmosphere • by several thousand years • (an indicator of melted ice sheets) • Implies that pCO2 is a primary driver of melting. • Alternatively, pCO2 could be driven by changes in • meteorological forcing: • dust delivery of trace metals to the ocean surface • an acausal correlation between Northern Hemisphere summer insolation and ice volume
Archer et al, 2005 “Because CO2 is more soluble in colder water, colder sea surface temperatures could lower pCO2. However, the magnitude of the glacial cooling can account for only a small fraction of the observed pCO2 drawdown.”
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry - Mechanisms to lower glacial pCO2: • Increase biological activity at surface • so that Carbon sinks to deep sea • sediments as particles • Increase Ocean Inventory of PO43- and NO3- • Change the ratio of nutrient to C in phytoplankton • Iron limitation of biological production at surface indicates a Southern Ocean Biological Pump could have intensified in a dustier, more iron-rich environment. • Glacial dust could stimulate the rate of Nitrogen fixation, increasing the ocean pool of NO3-
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry - Mechanisms to lower glacial pCO2: • 2. Change the pH of the whole ocean • Convert seawater CO2 into HCO3- and CO3=, which can’t evaporate in the atmosphere. • pH is regulated by balance between influx of dissolved CaCO3 and removal by burial of CaCO3 sediments. • Timescale of 5-10 kyears is within observed timescales.
Archer et al, 2005 • 2. Change the pH of the whole ocean • Conditions under which it could occur: • Glacial rate of weathering is higher • CaCO3 deposition shifts to deep sea • Rate of CaCO3 production decreased • CaCO3 compensation may also affect pCO3 response to the biological pump in #1. • Results: • burial efficiency would increase • the Ocean would become more basic • degradation of biological C in sediments would promote Calcite dissolution, further increasing Ocean pH.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry - Mechanisms to lower glacial pCO2: Two Caveats: “The ocean carbon cycle is a complicated system, controlled by biological processes we are only beginning to understand. Thus the formulation of the model is not completely con-strained by our understanding of the underlying processes. Furthermore, we use the model to predict…conditions which we are unable to observe except indirectly via clues preserved in the sedimentary record.”
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry - Mechanisms to lower glacial pCO2 - CO2 pump scenarios: • Fe fertilization of existing NO3 or PO4 pools • attains glacial pCO2 values in box models • But not in circulation models • 2. Increase NO3- by 50% • Attains glacial pCO2 for a few thousand years until CaCO3 compensation lowers Ocean pH. • Requires a change in the Redfield Ratio.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry - Mechanisms to lower glacial pCO2 - CO3= pump scenarios: • Coral reef hypothesis: lowered sea level causes a decrease in shallow CaCO3 deposition, which drives increased deposition in the deep sea • Increased pH would lower pCO2 • Not backed up by deep-sea cores • Rain ratio hypothesis: decrease in CaCO3 production or in-crease in organic carbon production could shift Ocean pH. • A doubling of H4SiO4 could explain it, but can’t be rationalized. • Predicted distribution of CaCO3 on seafloor is a poor fit.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry – Procedures and summaries: • Present Day Ocean simulation • pCO2 within 2 μatm of observed values • Distribution of CaCO3 a poor fit: Present-day CaCO3 distribution on seafloor Modeled Present-day CaCO3 distribution on seafloor
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry: • The Glacial Ocean model description: • High Lat. Air temperatures 10°-15° C colder than now • Tropical cooling 1°-2° C cooler from plankton and O isotope ratios • Glacial flow field estimated from best “second guess” velocities • Atlantic overturning shallower and 30% slower than now • δ13C tracer says Southern Ocean was high-nutrient, low Oxygen, contradicting Cd data.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry: • The Glacial Ocean model results: • Iron flux to sea surface increases by 2.5 goes to regions that already receive sufficient iron. • NO3- decreases from 110 x 1012 mol to 80 x 1012 mol. • pCO2 lowered by 8 μatm. • CO3= and H4SiO4 tweaked until burial rates of CaCO3 and SiO2 are those of present day. • 17% H4SiO4 decrease yields a 70% SiO2 burial increase. • Organic C production increased from 0.198 to 0.210 • Acidification of ocean overwhelms iron fertilization, increasing pCO2 to 280 μatm.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry: • Collapse of the terrestrial biosphere: • 13C/12C ratio in deep sea CaCO3 was .4%o lower, indicating that an isotopically-depleted carbon reservoir released 40 x 1015 mol C, raising the Ocean-Atmosphere inventory by 1% • Possible sources: • Terrestrial biomass: 40 x 1015 mol C • Soil organic carbon: 120 x 1015 mol C • Sedimentary C on continental shelves
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry: • Collapse of the terrestrial biosphere: • Reconstructions call for 2-3 x this δ13C value. • Initially raises pC02 to 305 μatm. • Reaction with CaCO3 will neutralize the added CO2 • Lowering to 297 μatm predicts a lowering of 17 μatm in the future. • After compensation, pCO2 is 295 μatm.
Archer et al, 2005 A New model of Ocean and Sediment Geochemistry: • Tropical Temperatures • Lowering Tropical Sea Surface Temperature by 4°C decreases pCO2 by 5 μatm. • Biological production is altered • Stratification decreases, organic Carbon increases. • SiO2 decreases as H4SiO4 recycling decreases. • Small increase in pCO2.
Archer et al, 2005 • Constraints on the cause of glacial/ interglacial atmospheric pCO2 • Deglacial increase leads ice volume, eliminating sea-level-driven explanations such as submersion of continental shelves • Deglacial transition was slow: 6-14 kyears. • The pCO2 response is much faster. • Glacial rates of weathering and burial • were not much different than today. • Isotopic signatures of C, N, B, Cd, Ba • Distribution of CaCO3 and SiO2 on sea floor
Archer et al, 2005 Solution: challenge one or more of the basic assumptions of chemical oceanography! • Ocean circulation models are more diffusive than the modern ocean, underestimating the pCO2 sensitivity to the biological pump • Increase the glacial NO3- inventory beyond the PO43- limitation, assuming the Redfield N/P number was different in glacial time. • Double the inventory of H4SiO4 in the ocean, raising the pH of the deep ocean.
USMAI (all campuses) Number of hits Request permutation (No Adjacency) 0 Words= Greenhouse Puzzles Part II Prince George’s Memorial Library System: Keyword Search: ti:(Greenhouse Puzzles, Part II) 0 record(s) found.
Greenhouse puzzles Part 2Secondary sources: A silicon-induced “alkalinity pump” hypothesis, Marine Inorganic Chemistry/ Department of Chemical Oceanography, The Ocean Research Institute ORI, University of Tokyo, Japan http://www.ori.u-tokyo.ac.jp/en/special/topics_4/topics-e.htm (refers to Broecker and Peng, Part 2 1994 version as “Archer’s World”. Also references Martin, J.H., “The Iron Hypothesis”) Field-based Atmospheric Oxygen Measurements and the Ocean Carbon Cycle, PHD Thesis by Britton Bruce Stephens, Chapter 6, The Influence of Antarctic Sea Ice on Glacial-Interglacial CO2 Variations Modeling of marine biogeochemical cycles with an emphasis on vertical particle fluxes, PhD Theis by Regina Usbeck, http://www.awi-bremerhaven.de/GEO/Publ/PhDs/RUsbeck/RUsbeck.html Zeng, Ning, Glacial-Interglacial Atmospheric CO2 Change - the Glacial Burial Hypothesis. http://www.atmos.umd.edu/~zeng
Greenhouse puzzles Part 2Secondary sources: • ORI: biological pump model of atmospheric CO2 variability • Stephens: Harvardton-Bear index: • Actual atmospheric CO2 change / • potential change due to cooling of low-latitude surface box • Usbeck: compares others’ works with recent estimates of total Corg • accumulation • Zeng: Ocean δ13C, .35%o, • land-carbon difference (Holocene - LGM) 460
Substitute or correct paper? The sequence of events surrounding Termination II and their implication for the cause of glacial-interglacial CO2 changes Wallace S. Broecker and Gideon M. Henderson, Paleoceanography, V 13 , No 4, PP. 352-364, August 1998 Wallace Broecker, Lamont-Doherty Gideon Henderson, now at Oxford
Broecker and Henderson, 1998 Clues from the Vostok ice core: Antarctic Temperature and atmospheric CO2 increased together for 8000 years, bounded by A drop in dust flux at the onset A drop in δ18O at the finish A similar lag between dust flux and foraminiferal δ18O in the Southern Ocean indicates that the δ18O in Vostok ice is a valid proxy for ice volume. Synchronous changes in CO2 and Southern Hemisphere temperatures preceded melting of Northern Hemisphere ice Nutrient reorganization in North Atlantic occurs with or after the sea level rise
Broecker and Henderson, 1998 • Clues from the Vostok ice core: • The previous observations eliminate many scenarios proposed to explain the CO2 rise • Those which rely on sea level change • Conveyor-related nutrient redistribution • North Atlantic cooling • Southern Ocean scenarios become the front runners. • The most popular, Iron fertilization, has 2 problems: • Much of the dust demise occurs prior to the change in CO2, so there must be a threshold value above which it does not increase. • The CO2 rise continues for 4-5 kyr after the dust flux has fallen to zero.
Broecker and Henderson, 1998 • Clues from the Vostok ice core: • Problems with iron fertilization causing the rise in CO2 may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during Glacial periods. • In this case, residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon • This would explain the slow rampup of atmospheric CO2.
Broecker and Henderson, 1998 Timing is everything for Broecker and Henderson. More comfortable than their predecessors with relating time markers, their whole theoretical setup is based on these time relationships. O2 created by photosynthesis has the Isotopic composition of surface seawater, which is controlled by global ice volume. Turnover time is 1-2 kyears. Therefore, δ18Oatmshould have risen 1.4%owithδ18Oocean.
Broecker and Henderson, 1998 The first assumption is that variation in ocean surface δ18O is the only contributor to changes in δ18Oatm. They then claim that the Dole Effect, where the atmosphere is enriched in 18O by 23.5%o over the ocean, keeps it steady. They then present similar offsets between events as indicating a good correlation.
Broecker and Henderson, 1998 • Broecker’s Bipolar Seesaw concept is also an important consideration, where deepwater formation alternates between the North and South Atlantic. This eliminates mechanisms that occur only in the North Atlantic. • Cooling in the Southern Ocean at the same time as CO2 is falling is considered as a cause, but is nowhere strong enough to cause the observed drop. • Changing the productivity or alkalinity is also suggested as a control of Oceanic CO2. Observations indicate that these changes moved in the opposite direction. • Nitrogen fixation by iron fertilization is considered, but the residence time for NO3 is too long to keep it locally confined.
Broecker and Henderson, 1998 Tentative conclusions: • δ18O constrains the rise in atmospheric CO2 to have preceded the melting of the North American ice sheets. • This eliminates seal level change, North Atlantic Nutrient redistribution, and North Atlantic cooling as causes. • Iron fertilization can’t explain • Southern Ocean paleoproductivity • the long duration of the CO2 rise • Increased dust flux in the glacials caused more nitrogen fixation, • which allowed a greater CO2 drawdown in surface waters. • Long residence time of NO3 in ocean explains how CO2 can continue to increase after the dust flux ix zero, and means productivity changes can be global.
Newer ideas 1: Zeng, Ning, Glacial-Interglacial Atmospheric CO2 Change - the Glacial Burial Hypothesis Readily available from http://www.atmos.umd.edu/~zeng
Newer ideas 1: Zeng, N Advancing ice sheets buried vegetation and soil carbon accumulated during warm periods. Simulation over 2 cycles found a 547 Gt carbon release, resulting in a 30 ppmv increase in atmospheric CO2, the remainder absorbed by the Ocean. Atmospheric δ13C drops by .3%o at deglaciation, followed by a rapid rise to a high interglacial value, in response to oceanic warming and regrowth on land. With other ocean-based mechanisms, offers a full explanation of the observed atmospheric CO2 change.
Newer ideas 1: Zeng, N Fig. 8. Modeled atmospheric CO2 (a) and land carbon storage (b) from the control run and 5 sensitivity experiments described in the text: control is in black line, SST in green, CO2v120 in yellow, SoilD5h in red, Soil D20k in blue, and WarmGlac in purple. The largest change of a 55 ppmv deglacial CO2 increase is due to a cooler glacial ocean in addition to the land carbon release (green) and a 40 ppmv increase due to a long delayed regrowth (blue).
Newer ideas 1: Zeng, N Data from Table 1: Land carbon difference, Holocene - LGM
Newer ideas 1: Zeng, N • Look for it: • On the ground • Back in time • In the models • In comparisons
Newer Ideas 2: Paillard and Perenin • Glacial bottom waters • were possibly much more saline • May have an unsuspected large density • Glacial deep stratification could account for the difference. • Ice formation around Antarctica involves • Brine rejection over the Continental Shelves • Is directly linked to changes in • Sea Ice Formation • Antarctic ice-sheet extent
Kitchen Experiment: mixing saline water Cold 30% salt water They mixed immediately. Warm 10% salt water
Newer ideas 3: Toggweiler, JR, GFDL, Climate change from below Quaternary Science Reviews 24 (2005) 511-512
Newer ideas 3: Toggweiler, Climate Change from below • Adkins et al, 2002, showed bottom waters around Antarcti-ca are significantly saltier than the rest of the ocean, appar-ently from accumulation of brine during sea ice production. • This shows that the glacial deep ocean was more stably stratified than it is today. • Geothermal heat would have slowly warmed it from below, destabilizing it, like a discharging capacitor. • Just 2°C is enough to destabilize it. • This would take 10,000 years. • This matches: • Heinrich Events in the North Atlantic. • Bond cycles in Greenland Ice Cores • Bi-polar seesaw between Greenland and Antarctica • He gives several examples separated by 7000 years.
Newer ideas 3: Toggweiler, Climate Change from below • This injects salt into the upper North Atlantic, kick-starting thermohaline circulation. • Reinvigorated circulation warms up Greenland and the North Atlantic. • This confirms the finding that the warmest intervals in Greenland occur during the interstadials that follow Heinrich and Antarctic Intervals. • This contradicts the prevailing view • these events are caused by fresh water input • Explains why interstadials after Heinrich events are longer and warmer than others.
Newer ideas 3: Toggweiler, Climate Change from below • Short paper. • Is it the right one? • So I wrote and asked! Woah! Preprint!
Newest ideas yet: Toggweiler et al • Another new idealized general circulation model explains: • tight correlation between atmospheric CO2 and Antarctic temp • lead of Antarctic temp over CO2 at terminations • Shift of ocean’s δ13C minimum from N. Pacific to Atlantic sector of the southern Ocean Changes occur at transitions between on and off states of the southern overturning circulation.