Geochemical fingerprints of the ice-age (Southern) Ocean

Geochemical fingerprints of the ice-age (Southern) Ocean THE SOUTHERN OCEAN, ITS DYNAMICS, BIOGEOCHEMISTRY AND ROLE IN THE CLIMATE SYSTEM NCAR, Boulder, CO 10-13 April 2017 Bob Anderson

Motivation: Ice core records reveal tight coupling between CO 2 and climate Why? Brook, NATURE Vol 453 15 May 2008

Take-home messages from this presentation 1) The biological pump was more efficient during the last glacial period, lowering the oxygen concentration in the deep sea. 2) Carbon was released from the deep ocean during deglaciation via the Southern Ocean 3) Major shifts in the SWW during deglaciation 4) Challenge did the winds drive CO 2 release?

Deep-ocean CO 2 storage represents a balance between biology and physics CO 2 Biological Pump CO 2 Preformed Nutrients CO 2 &Nutrients Respiration Atmospheric CO 2 reflects a global balance between biological drawdown and physical ventilation. Figure of K Speer redrawn by T Trull

Where and How was CO 2 stored in the deep ocean? Guiding principle: Any biological pump mechanism for lowering iceage pco 2 atm decreases the dissolved O 2 content of the ocean interior Sigman et al., 2010, summarizing one of the main points from Broecker, 1982. C 106 H 175 O 42 N 16 P + 150 O 2 106 CO 2 + 78 H 2 O + 16 HNO 3 + H 3 PO 4

How do we assess changes in [O 2 ]? There is no direct geochemical proxy. Therefore: DO 2 constrained indirectly: Sediment redox state (measure U, Re); depends on: a) Bottom water [O 2 ] (oxygen supply), b) Organic carbon supply (Measure xsba, opal) Infer: Bottom water [O 2 ]

Central Equatorial Pacific Illustration ML1208-17PC Planktonic δ 18 O ( ) TT103-PC72 Ba xs Flux (mg cm -2 kyr -1 ) 0 50 100 150 200 250 300 350-2.5 3-1.5-0.5 1.4 1 0.6 0.2 Age (ka) 3 0 50 100 150 200 250 300 350 Age (ka) PhD results of Allison Jacobel, LDEO Geochemical fingerprints of low bottom water oxygen during ice ages 3.5 4 4.5 5 260 220 180 0 1 2 LR04 Benthic δ 18 O ( ) Antarctic pco 2 (ppm) ML1208-17PC au (ppm)

Compelling qualitative evidence for the Pacific Ocean Earth and Planetary Science Letters 277 (2009) 156 165 Earth and Planetary Science Letters 299 (2010) 417 425 Nature Geoscience 5 (2012) 151 155

Compelling qualitative evidence for the Atlantic Ocean Nature Geoscience 8 (2015) 40-43 North Atlantic Nature Communications 7 (2016) doi 10.1038/ncomms11539 South Atlantic Nature 530 (2016) 151 155 Southern Ocean

Deep N Pacific (>5000m): Magnetic minerals lost from ice-age sediments due to low BWO Other types of geochemical fingerprints of low BWO Korff et al., 2016, Paleoceanography 31: 600-24

Low-oxygen waters upwelled in the Southern Ocean during the ice ages Core site in the Amundsen Sea Modern Ocean Oxygen Lu et al., 2016, Nature Communications 7: doi 10.1038/ncomms11146

Low-oxygen waters upwelled in the Southern Ocean during the ice ages Low I/Ca ratios of planktonic foraminifera (geochemical fingerprint) indicate very lowoxygen water in the ice-age subsurface Amundsen Sea Lu et al., 2016, Nature Communications 7: doi 10.1038/ncomms11146

Low-oxygen waters upwelled in the Southern Ocean during the ice ages Ice-age O 2 concentrations in CDW must have been < ~ 20 µmol/kg for reduction of IO 3- to I -. Lu et al., 2016, Nature Communications 7: doi 10.1038/ncomms11146

Physical changes in the Southern Ocean proposed to allow low-oxygen conditions

Physical changes in the Southern Ocean proposed to allow low-oxygen conditions Ice-age expansion of deep overturning cell: a) isolated deep waters, allowing low oxygen b) accompanied by northward shift in upwelling Watson et al., 2015, Nature Geosci 8: 861-4

Opal (diatom frustules) burial traces shift in locus of upwelling APF Dissolved silicic acid section along the prime meridian WOA09 and ODV

Opal (diatom frustules) burial traces shift in locus of upwelling APF Maximum opal burial in modern sediments south of the APF Reflects Si supply to diatoms Geibert et al., 2005, Glob Biogeochem Cycles 19: GB4001 doi:10.1029/2005gb002465

Opal (diatom frustules) burial traces shift in locus of upwelling Core sites spanning modern [Si] gradient used to investigate ice-age conditions

Opal (diatom frustules) burial traces shift in locus of upwelling Opal burial (mmol Si/m 2 /yr) 350 300 250 200 150 100 50 0 APF Holocene Glacial 40 45 50 55 Latitude S Peak opal burial shifted ~5 Northward during ice ages Kumar, Anderson et al., 1995. Nature 378: 675-80 with newer results

Competing hypotheses to explain northward shift in opal belt during ice ages 1) Upwelling remained unchanged expanded sea ice inhibited plankton south of the APF unused nutrients mixed northward prior to consumption Charles et al., 1991, Paleoceanography 6: 697-728

Competing hypotheses to explain northward shift in opal belt during ice ages 1) Upwelling remained unchanged expanded sea ice inhibited plankton south of the APF unused nutrients mixed northward prior to consumption Charles et al., 1991, Paleoceanography 6: 697-728 Disproven by N isotopes (talks by Adkins and Sigman) Nutrients were utilized efficiently south of the APF

Competing hypotheses to explain northward shift in opal belt during ice ages 1) Upwelling remained unchanged expanded sea ice inhibited plankton south of the APF unused nutrients mixed northward prior to consumption Charles et al., 1991, Paleoceanography 6: 697-728 Disproven by N isotopes (talks by Adkins and Sigman) Nutrients were utilized efficiently south of the APF 2) Upwelling center displaced northward (and most upwelled water mixed northward?) Geochemical fingerprints indicate increased nutrient utilization (efficiency of the biological pump) throughout the Southern Ocean and low oxygen in the deep sea.

Maximum upwelling south of the modern APF coincided with deglacial rise in atmospheric CO 2 - Winds invoked HS1 TN057-13 53.17 S SUMMARY OF EVIDENCE: Opal burial traces upwelling: Diatoms use available Si, Deglacial increase in opal burial traces southward shift in upwelling and supply of nutrients Peak opal burial exceeds anywhere in modern ocean (No modern analog during So Ocean reorganization) Modified from Anderson et al., 2009

Geochemical fingerprints of CO 2 ventilation are consistent with interpretation of opal flux Sea-air DpCO 2 Opal flux Atlantic Southern Ocean Eastern Equatorial Pacific Planktonic d 13 C Atm pco 2 Atm d 13 CO 2 Martínez-Botí et al. Nature 518, 219-222 (2015) doi:10.1038/nature14155

Deglacial So Ocean upwelling injected nutrient-rich waters into the thermocline (AAIW) Poggemann et al. 2017. Earth and Planetary Science Letters 463: 118-26

Deglacial So Ocean upwelling injected nutrient-rich waters into the thermocline (AAIW) Cd w (nutrient tracer) increases abruptly during HS1 at 850m (AAIW) but not below 1300m (UNADW), reflecting nutrient injection in the Southern Ocean Poggemann et al. 2017. Earth and Planetary Science Letters 463: 118-26

Deglacial So Ocean upwelling injected nutrient-rich waters into the thermocline (AAIW) Pattern of AAIW nutrient injection (Cd w fingerprint) matches the opal tracer of upwelling in the Southern Ocean Poggemann et al. 2017. Earth and Planetary Science Letters 463: 118-26

What physical forcing was responsible? Wind? Buoyancy flux? Watson et al., 2015, Nature Geosci 8: 861-4

Evidence for shifting winds WAIS Divide Ice Core Excess deuterium (d ln, a geochemical fingerprint of moisture source conditions) changes abruptly with each NH climate oscillation implicating shift in winds with each NH abrupt climate change. Greenland warming Greenland cooling Markle et al., 2017, Nature Geosci 10: 36-40 Composite records

Evidence for shifting winds Patagonian Glaciers Rapid retreat 18-16 ka (2013) Scientific Reports 3: 2118; DOI:10.1038/srep02118 See also Denton et al., 1999, Geografiska Annaler Series a-physical Geography 81A: 107-53 Hall et al., 2013, Quat. Sci. Rev., 62: 49-55. Composite records

Evidence for shifting winds New Zealand Glaciers Rapid retreat 18-16 ka Required rapid warming Inferred southward SWW Expansion of the subtropical gyre Putnam et al., 2013, Earth and Planetary Science Letters, 32: 98-110. Linked to contemporary changes south of Australia

Evidence for shifting winds Subtropical species spread S of Australia during Heinrich Stadials local warming and displacement of currents

Evidence for shifting winds Tristan da Cunha: Fossil evidence in bog sediments for displacement of storm tracks Ljung et al., 2015, Quaternary Science Reviews 123: 193-214

Evidence for shifting winds 10 15 20-10 5 25-40 -30-20 Megg's Hill Peatland, 51 S C27-based C29-based C31-based Feb. 2015 peatland water Feb. 2015 precip. median 0 TN057-13-4PC, 53.2 S 0 5 10 15 Age, cal. ka 20 25 Opal Flux, g cm 2 yr 1 1 2 3 4 5-50 Jon Nichols, LDEO Unpublished 0 δdp, VSMOW Aukland Island: Leaf wax isotopes in bog sediments trace large shift in moisture source (winds)

Summary of the ice-age ocean and deglaciation 1) Ice-age: Efficient biological pump, low oxygen in the deep sea 2) Ice-age: Locus of So Ocean upwelling located north of its present position. 3) Deglaciation: Upwelling shifted south Released CO 2 Injected nutrients into AAIW (thermocline) 4) Deglaciation: Winds shifted south 5) Challenge: Did the winds play a role in forcing So Ocean changes?

Redox state: Exploit trace elements (uranium) precipitated under anoxic conditions Seawater [U] [O 2 ] [U] ~Constant Depth in pore water z Ua precipitation U a = F Ud MAR = D S( [U]/ z) MAR Variable z decreases, and Ua increases as [O 2 ] decreases or Corg flux increases

Lowering [O 2 ] or increasing C-org rain raises Ua Seawater [U] [O 2 ] [U] ~Constant Depth in pore water z Ua precipitation U a = F Ud MAR = D S( [U]/ z) MAR Variable z decreases, and Ua increases as: [O 2 ] decreases or Corg flux increases

Plausible scenario for the ice-age Pacific Ocean But not based on any quantitative O 2 estimates Black = Modern observations Orange = Plausible LGM Jaccard et al., 2009

Carbon must have been transferred to the deep ocean during the ice ages The deep ocean is: 1) The only C reservoir large enough to accommodate 200 GtC from the atmosphere during each peak ice age... 2)...and a much larger inventory of carbon released from the terrestrial biosphere. 3) The only large C reservoir capable of exchanging carbon with the atmosphere as rapidly as indicated by the ice cores.