Difference between present Antarctic sea ice and LGM sea ice

Ice Sheet Retreat Ice Sheet retreat begins about 18 -14 kyrs ago, and are largely gone by 6,000 years ago.

How thick were the ice sheets? Two models – thick (early) and thin (more recent). Thickness determination is difficult: 30% of ice is buried below level plane. Thicknesses are now contrained by sealevel estimates and new ice flow (cemented vs free base) models.

This ‘rebound’ from past glacial loading can confuse measurements of present sea level change.

How do we determine the volume of the LGM Ice Sheet? From sea level rise; 2. Glacial moraines give lateral extent (but not thickness) 3. ‘Rebound’ of the depressed continent beneath the ice sheet (cm/year) can be used to estimate thickness. ice 70% 30% continent

Hudson Bay paleo-beach Post-glacial rebound. If bottom of ice sheet is depressed (buried) below the initial level surface, and then the ice sheet melts, the land mass will rebound to the original height. 14C dating of the old beach marks (as the land rose) allow estimates of this rebound rate. 150 m/7000 yrs = 2 cm/year.

Ice Sheet Volume

Sea Surface Temperature (today)

Sea Surface Temperature Change at LGM

Other Estimates of SST Change at LGM Alkenone content of pelagic Plankton d18O of CaCO3 pelagic forams Alkenones and d18O indicate tropical SST decreased by ~2 to 4ºC - at Last Glacial Maximum

ALKENONES; a proxy for seawater temperatures – without needing to know ice volume! What is an alkenone? A saturated fat used by phytoplankton (for cell walls, interior fluid). The degree of saturation (number of carbon-hydrogen bonds) in these fats depends on temperature. HIGH saturation fats become solid at low temperatures (like lard). So extraction of these alkenones from plankton in sediments (specific species), and measurement of their degree of ‘saturation’, can give the temperature at which the formed. e. Huxleyi everyones favorite

What is a saturated fat?

transfat – Not climate related Saturated fat Unsaturated fat Olive oil

Alkenones collected from sediment cores as function of latitude (world wide). Previous work (Geochim. Cosmochim. Acta 62: 1757-1772, 1998) showed that values for the alkenone unsaturation index UK'37 measured in Modern sediments throughout the open World Ocean strongly correlate with annual mean sea-surface temperature (SST).

So by Picking out individual species of phytoplankton from sediment cores. extraction of alkenones (using organic solvents) from these skeletons Running the extractions in a gas chromatograph – to get the specific fat saturated to unsaturated ratio It is possible to get sea water temperatures (benthic or pelagic) when the phytoplankton grew WITHOUT THE NEED TO KNOW ICE VOLUME.

ALKENONE record of SST off Santa Barbara, CA Comparing d18O with alkenone seawater temperatures

So if you do alkenone extraction and analysis from sediments (i. e So if you do alkenone extraction and analysis from sediments (i.e., take a series of cores along a profile along a longitude) that are LGM in age in the tropics, you can estimate the sea surface temperature – in the tropics – at LGM time. And that is quite small (i.e., the equator didn’t get very cold at LGM although the poles and intermediate latitudes did).

Meridional temperature distribution. Remember the Cretaceous? If the tropical SST temperature during the LGM was only a few 0C LOWER during the LGM than the present, and the poles (Antarctic, Greenland) were 10 to 200 C lower, how would the LGM curve correspond to the graph below? N Pole S Pole

DUST – times of large continental ice sheets produced abundant dust! Glacial periods were drier, colder air temperatures implies reduced moisture and reduced rainfall (but not everywhere; i.e., southwest U.S.). Less vegetation cover. Glacial periods had higher winds. (think meridional temperature gradients) Ice Sheets and mountain glaciers produced lots of ‘rock flour’; fine silt. The higher winds, drier climate and fine silt combined to produce abundant DUST – which was transported on global scale. Indian Ocean sediments indicate that LGM dust levels were 5 x higher than present – in that area. Loess deposits (wind driven silt) from the LGM time. Large, thick loess deposits exist in Eastern Washington and are responsible for fertile wheat fields there.

Desert dust source regions today – Arrows are prevailing winds today: during LGM, these areas produced even MORE dust than they are today. Note: dust from the Sahara desert is blown out into the south Atlantic, providing IRON that fertilizes upper ocean productivity

Regions with abundant sand dunes during: Top – today. Bottom – LGM time. Dust levels in Antarctica were 10 x those today, as estimated from the ice cores. If surface ocean biology (diatoms vs coccolithophores) plays a role in the transition from glacial to interglacial periods… By INCREASING the biological PUMP it is likely to be through ocean circulation – and dust (as a nutrient supply).

Climate change near the North American Ice Sheets: Near the edge of the ice sheets, the climate was much wetter than present; with Lake Bonneville (covered 40% of the State of Utah) being an example. Lake Bonneville existed about 15K years ago, and drained catastrophically into the Columbia River when the natural dam in the north failed. This drainage may (or may not) have produced changes in the ocean circulation in the NE Pacific Ocean – along with Lake Missoula floods. In contrast to the SW, the Pacific NW was colder and drier, and many areas were desert.

Jet stream in modern times: note it is just north of Seattle. Jet stream (from numerical models) during Last Glacial Maximum: note that it is considerably farther south.

Cross-section of bottom water formation in the North Atlantic. The N - S transfer of water via ocean circulation is responsible for significant transfer of solar heat, from the equator (high input zone) to high latitudes. Any change in this circulation pattern results in a change in climate in the temperate and polar regions

“Ventilation” of the oceans: 14C dating of DIC (dissolved inorganic carbon) 14C age dating of seawater means - “when was the Dissolved Inorganic Carbon in the water last in equilibrium with the atmosphere?” Generalized circulation of the oceans (now) – Deep-water formed in the N. Atlantic: (zero 14C age). By the time it gets to the NE Pacific (as bottom water), about 1500 years have passed.

HCO3– (1777 mmol/kg) and CO3= (225 mmol/kg). So mostly bicarbonate. 14C dating of DIC (dissolved inorganic carbon) Dissolved inorganic carbon in seawater. HCO3– (1777 mmol/kg) and CO3= (225 mmol/kg). So mostly bicarbonate. Some of these carbon atoms are the isotope 14C, which is formed in the atmosphere from cosmic ray bombardment, and decays which with a half life (50% gone) of 5,730 years. Probably can measure out to 6 half-lives, or 30,000 years. DIC can be obtained from water samples taken from the top and bottom of the water column. By carefully measuring the “14C age” of this DIC and comparing surface and benthic values, it is possible to get an estimate of the number of years since that bottom water was exposed on the surface.

Ocean convection cell: if convection is FAST, then surface and deep water will have (about) the same values of 14C. If the convection cell is slow (stopped), surface and bottom water will have very different 14C values. 14C Young 14C Old (decayed) 14C

By measuring the 14C values in sediments in benthic and pelagic forams – as a function of sediment age, it is possible to estimate the ‘vigor’ of ocean circulation at different times (i.e., during the present, and during the LGM). In the Pacific, the age difference (benthic vs pelagic) is similar to today; In the equatorial Atlantic, the age difference was about twice (675 years) the modern value of 350 years. This means bottom water circulation in the Atlantic at LGM was slower than today – i.e, less bottom water formation at high latitudes. This means that during the LGM, less thermal energy was transferred from the equatorial Atlantic to the north Atlantic; with strong implications for climate in the northern hemisphere. The area around Paris, for instance, became arctic tundra.

Deep Water Formation: Present vs LGM

Possible Impact of Reduced NADW Formation Rates on Air Temperatures

Increased Global Aridity at LGM Ice Core Record of Dust - increased dust at LGM due either to increased strength of winds or dust source (aridity)

Vegetation Changes NOW Use pollen records from several lakes to reconstruct regional vegetation distribution during LGM. LGM

Atmospheric Gases during LGM CO2 was 180 ppm (vs 280 ppm at warm interglacials) CH4 was ~350 ppb (vs 700 ppb at interglacials)

Summary: Climate Conditions during LGM Insolation rates about the same as today. Colder (~ -4 º C globally and ~ -10 ºC near the poles (maybe colder) and ~-2 to -3 ºC in tropics). Ice Sheet volume was ~ twice today. Sea Level lower by ~ 125m. Drier and dustier (globally). Reduced atmospheric CO2 and CH4 levels Vegetation more arctic like (tundra, steppe). Deep Ocean circulation more sluggish.

Sea Level Rise • Use 14C and 230Th/238U to date the age of a sequence of submerged corals that lived close to the sea surface. • The rate of sea level rise has pulses. (14C ages are too young by up to ~3K yrs.)