Biogeochemical changes over long time scales

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1 Biogeochemical changes over long time scales Eric Galbraith McGill University, Montreal, Canada

2 Overview What is a long time? Long timescale observations from marine sediments Very quick look at biogeochemical changes during two time intervals: Paleocene-Eocene Thermal Maximum (hot) End of the last ice age (cold to warm)

3 How long is a long time? Biogeochemically Depends on the element defining timescale is residence time, τ r τ r = mass of element in reservoir output flux from all sources τ r = mass of element in reservoir input flux from all sources τ r = e-folding timescale

4 How long is a long time? Biogeochemically Example residence times: Atmospheric methane: ~ 10 y Ocean dissolved iron: Ocean fixed nitrogen: Surface carbon: ~ 50 y ~ 4,000 y >100,000 y

5 How long is a long time? Geologically 4.3 billion years If Earth history = 1 calendar year, ending now

6 The calendar year of Earth history January: Earth coalesces, intense meteor bombardment Early February: temperatures become low enough for liquid water, ocean forms

7 The calendar year of Earth history Early March: cyanobacteria are photosynthesizing Late June: atmosphere becomes oxygenated

8 The calendar year of Earth history Mid-october: First complex multicellular life

9 The calendar year of Earth history Mid-November: First calcifying organisms

10 The calendar year of Earth history November 26: First land plants and animals

11 The calendar year of Earth history December 10: Dinosaurs evolve

12 The calendar year of Earth history December 25: Dinosaurs go extinct

13 The calendar year of Earth history December 31 14:00 Early hominids appear

14 The calendar year of Earth history December 31 23:55 Homo sapiens sapiens

15 The calendar year of Earth history December 31 23:57 40 Last glacial maximum

16 The calendar year of Earth history December 31 23:58 50 Stabilization of current warm period (Holocene)

17 The calendar year of Earth history December 31 23:59 58 Industrial revolution

18 The calendar year of Earth history December 31 23: Satellite observations of Earth begin

19 Biogeochemical changes over long times scales There is a lot to choose from! First 11 months: extremely data limited, but fascinating links to evolution of cellular machinery Early December: scattered data in outcrops on land, often altered. Very different climate, tectonics, and organisms. Last week of December: seafloor sediments are still in place, preserve pristine global record. Continents close to modern positions, organisms similar, range of climates. Last 3 minutes of December: have accurate radiocarbon dating, allows synoptic global pictures. Organisms identical, ice age and modern climates.

20 Advantages/disadvantages of looking in the geological record Can see true climate signals, beyond noise of decadal/ centennial variability timescales of ocean biogeochemistry: e.g. surface carbon cycle residence time ~15 minutes of calendar year Many dramatically different climate states to explore But No direct observations, always relying on proxy evidence

22 Piston coring

24 Joides Resolution

26 Atlas of Benthic Foraminifera Foraminifera The paleoceanographer s Rosette: grow at a range of depths, record temperature, salinity, and chemistry Silvia Watanabe

28 Oxygen isotopes Stable oxygen isotopes (16,17,18) should be fractionated during calcite precipitation originally suggested by Urey, 1947 Soon verified in natural calcite samples, reported as δ 18 O (= 18 O: 16 O sample / 18 O: 16 O ref -1)*1000 Where ref is Pee Dee Belemnite (a natural carbonate rock). Applied to sediment samples by Emiliani in 1955

29 Oxygen isotopes A bit more complicated, for 2 reasons: δ 18 O of water also varies locally, due to changes in hydrological cycle, and globally, due to changes in ice sheets on land Foraminiferal calcite δ 18 O diverges from theoretical values, due vital effects. Luckily, some species seem more reliable targeted picking.

30 Carbon isotopes Stable carbon isotopes (12,13) are not fractionated much by temperature, but instead dominantly reflect the δ 13 C of dissolved DIC Reported as δ 13 C (= 13 C: 12 C sample / 13 C: 12 C ref -1)*1000 Where ref is Pee Dee Belemnite (a natural carbonate rock). Vital effects are larger for carbon than for oxygen, so very important to consider species Controls on δ 13 C of DIC: Air-sea exchange (temperature/ kinetic); biological pump (respired carbon has low δ 13 C); whole ocean δ 13 C, controlled by global carbon cycle

31 Zachos et al The Cenozoic (post-dinosaur) Temperature 0 12 δ 18 O (%o) Dec 26 Dec 31

32 Eocene Paleocene-Eocene Thermal Maximum 27 December, 11h 2900 m Ron Blakey

33 Southern Ocean ODP Site 690 Central North America Polecat Bench warming Sluijs et al., 2007

34 Paleocene-Eocene Thermal Maximum Rapid warming (thousands of years), evident in δ 18 O Accompanied by large, global shift in δ 13 C Only conceivable explanations involve a release of low δ 13 C carbon to the biosphere, such as methane Accompanied by massive dissolution of seafloor carbonates

35 Zachos et al., Science 2005

36 Current thinking on the PETM The observed degree of ocean acidification required a large input of carbon to the system (>2000 Pg, similar to anthropogenic input) The carbon isotope signal requires that a large input of methane (or similar) also occurred, exacerbating warming Neutralization of acidity, by dissolution of carbonates and weathering, appears to have taken a few tens of thousands of years

37 Zachos et al The Cenozoic (post-dinosaur) Temperature 0 12 δ 18 O (%o) Dec 26 Dec 31

38 Scotese

39 The last ice age Lasted for about 100,000 years (11 minutes), until its peak ~20,000 years ago (23:57 40 ) At its peak, sea level was 120 m lower humans could go pretty much anywhere in the world without having to get in a boat (except Australia) Changes in the Earth s orbit seem to have been the pacemakers of ice ages but relatively small, amplified by unstable feedbacks in the system Atmospheric pco 2 was ~30% lower than pre-industrial, a critical feedback on the cold temperatures. Almost entirely due to ocean storage, due to lower temperature and biogeochemical changes.

40 The last deglacial transition ~ 4 C

41 The bumpy deglacial ride Warming was not globally uniform Northern and Southern temperatures alternated, in a bipolar see-saw Melting of the continental ice sheets destabilized ocean circulation Changes in surface currents redistributed heat, sea iceatmospheric feedbacks accentuated these changes These also had large biogeochemical impacts

Southern perspective Southern warmings were accompanied by increased diatom export to sediments Inferred increased upwelling Inferred stronger

42 Southern perspective Southern warmings were accompanied by increased diatom export to sediments Inferred increased upwelling Inferred stronger Southern westerlies/ more rapid overturning Linked to CO 2 increase Anderson, Ali, Bradtmiller, Nielsen, Fleisher, Anderson and Burckle, Science 2009

43 McManus et al. Nature 2005 Northern perspective Northern cooling (HS1) was accompanied by a reduction in the flux of NADW to the deep Atlantic Northern warming (BA) saw a reinvigoration of NADW Opposite to Southern Ocean timing

44 Oxygen changes Oxygen integrates physical and biological processes Concern that anthropogenic warming will cause ocean deoxygenation, endangering marine ecosystems (particularly fish and coastal benthos) Four proxy types have been measured at many sites, interpreted individually New compilation of global records

45 Upper ocean: more hypoxia Deep ocean: better oxygenated Jaccard and Galbraith, in review

46 Local changes, no obvious change in ocean mean Jaccard and Galbraith, in review

47 Overall decrease in oxygen, except in deep Atlantic ocean and SE Pacific Jaccard and Galbraith, in review

48 Overall increase in oxygen, with local deviations Jaccard and Galbraith, in review

49 Oxygen changes Deglacial warming caused deoxygenation of the upper ocean (but an increase of oxygen in the deep ocean) Within glacial-interglacial change, discrete steps, linked to global ocean circulation changes Greatest extent of hypoxic waters appears to have occurred in the middle of the deglaciation, when warming was most rapid, rather than at peak temperature

50 Summary The geological archive offers a tremendous range of past climate states for study Can be used to gauge biogeochemical sensitivities, and for revealing processes/responses you might otherwise not have thought of Paleo-records will remain, throughout our lifetimes, the only observational constraints we have on climate timescales As the number of cores, proxies, and their groundtruthings expand, these constraints will become stronger

A bit of background on carbonates. CaCO 3 (solid)

A bit of background on carbonates CaCO 3 (solid) Organisms need both carbon dioxide and carbonate Kleypas et al 2005 The two pumps put CO 2 into the deep ocean The long term record of climate change Or: