Arctic Paleoclimates

Transcription

1 Arctic Paleoclimates

2 The geologic time scale [from the Geological Society of America, product code CTS004, compiled by A.R. Palmer and J. Geissman, by permission of Geological Society of America].

3 Paleoclimate records for the Quaternary (1.8 Ma to present) Ice cores from Antarctica and Greenland (GISP2, GRIP, EPICA). Greenland records go back to the Eemian, Antarctic records go back 450, ,000 years. Marine sediment cores from all over the world, covering the entire Quaternary and even longer Pollen, diatoms, plant and animal macrofossils preserved in lake seiments and peat bogs Loess deposits, tree rings (dendrochronology), speleothems (mineral formations in limestone caves) Geomorphic features, such as raised beaches, moraines and glacial erratics

4 Ice and ocean cores

5 Ice and Ocean Cores Ice cores contain records of temperature from oxygen isotopes ( 18 O). Other factors equal, the higher the 18 O concentration in the ice core, the higher the temperature of condensation of the water vapor that led to the precipitation, and hence the warmer the conditions. Records of accumulation can be obtained from the thickness of annual ice layers. Impurities in the ice, such as wind blown dust and soluble ions yield information on characteristics of the atmospheric circulation and land surface. Marine (ocean) sediment cores comprise mixes of continentally-derived material and microscopic marine organisms that rain out, or live and die in the sediments and accumulate year after year in layers. Oxygen isotope records are preserved, coming from the remains of calcareous organisms (planktonic and benthic foraminifera) preserved in sediments. The higher the 18 O concentration, the higher the global ice volume. Different species of planktonic and benthic foraminifera thrive in different environmental conditions so changes in the abundance of different types record environmental changes in the uppermost and deep oceans, respectively. Marine cores also contain information from inorganic markers, such as detritus carries and dropped by icebergs.

6 bio1903.nicerweb.com Foraminifera www-paoc.mit.edu

7 Radiocarbon dating: Based on the decay rate of radiocarbon ( 14 C) which has a half life of 5730 years Radiocarbon is produced in the upper atmosphere by neutron bombardment of nitrogen atoms. The neutrons are part of the cosmic ray flux. Plants and animals assimilate radiocarbon into their tissues via photosynthesis and respiration, with a radiocarbon content in equilibrium with that in the atmosphere. Equilibrium occurs as there is a constant exchange of 14 C as old cells die and are replaced. When organisms die, exchange and replacement of radiocarbon ends, activating a radioactive clock For various reasons, including variations in the cosmic ray flux, 14 C levels have varied over time, and this needs to be adjusted for if one is directly compare radiocarbon and calendar ages. Calibration curve for the radiocarbon dating scale [from Wikipedia]

8 Dendrochronology: Dating and paleoclimate analysis using data from tree rings. Limiting climate factors Cross dating Sources:

9 Palynology: The study of pollen Image of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), oriental lily (Lilium auratum), evening primrose (Oenothera fruticosa), and castor bean (Ricinus communis). Source:

10 A glacial erratic U.S. Geological Survey photo by Bruce Molnia

11 Chronology of the Quaternary

12 The Vostok ice core record The Vostok record shows at least 4 major global scale ice advances over the past 400,000 years. The inferred temperature time series from oxygen isotope records is highly correlated with the ice core record of atmospheric carbon dioxide concentration.

13 Milankovitch forcings: Pacemaker of the ice ages Variations in eccentricity, axial tilt and precession (the timing of the equinoxes) affect the solar flux striking the surface at different latitudes and at different times of the year. These forcings have paced the timing of the major ice ages and interglacials of the Quaternary. Source: /~geol445/hyperglac/time1/milankov.htm

14 Past and future Milankovitch cycles. ε is obliquity (axial tilt). e is eccentricity. ϖ is longitude of perihelion. esin(ϖ) is the precession index, which together with obliquity, controls the seasonal cycle of insolation. Q day is the calculated dailyaveraged insolation at the top of the atmosphere, on the day of the summer solstice at 65 deg. N latitude. Benthic forams and Vostok ice core data show two distinct proxies for past global sea level and temperature. The vertical gray line is current conditions, at 2 ky A.D. [from Wikipedia].

15 Temperature and atmospheric greenhouse gas concentrations have closely followed each other for hundreds of thousands of years. Rises and falls in temperature precede greenhouse gas changes. This tells us that greenhouse gases operate as a feedback, globalizing the effects of Milankovitch forcings. Today s carbon dioxide concentration (about 390 ppm) is higher than anything seen in ice core records.

16 The last glacial cycle

17 The SPECMAP (Spectral Mapping Project) composite chronology for a set of seven stacked (superimposed) δ 18 O records from different ocean basins of the world. Dating involves tuning of the marine isotope records by orbital forcing [from Martinson et al., 1987, by permission of Elsevier]. Negative excursions in the δ 18 O record correspond to cold periods and positive excursions correspond to warm periods. The last interglacial, known as the Eemian (Isotope Stage 5.5), peaked about 125,000 years ago. The last glacial maximum (Isotope Stage 2.2) occurred 18,000-25,000 years ago depending on region. We are presently in another warm interglacial, and are scheduled to slide into another ice age in 30,000 years or so.

18 Modeled extent and elevation of the Greenland ice sheet during the Eemian interglacial (a-c) under three different temperature reconstructions (d) based on the GRIP ice core records [adapted from Cuffey and Marshall, 2000, by permission of Nature]. The ice sheet was much smaller during the Eemian and sea level was likely 4-6 m higher than today.

19 Summer surface air temperature anomaly over the Arctic (left) and extent and thickness for the Greenland ice sheet during the height of the Eemian interglacial from a multi-model and multi-proxy synthesis [from the IPCC-AR5, Working Group 1 Report, Figure 6.6]. The temperature plot is for the height of the Eemian minus pre-industrial.

20 Extent of Northern Hemisphere glacial ice during the Last Glacial Maximum (LGM) [from Denton and Hughes (eds.), 1981, by permission of John Wiley and Sons]. There were ice sheets over both North America and northwestern Eurasia. Global sea level was around 120 m lower than today.

21 Ice extent in northwestern Eurasia during the LGM [adapted by Siegert et al., 2001 from Svendsen et al., 1999, by permission of John Wiley and Sons]. Glacial ice covered parts of the continental shelves.

22 North Atlantic marine core records for the last glacial cycle contain layers of icerafted debris dropped by armadas of icebergs, the most notable termed Heinrich Events (H1-H6 above). These relate to rapid climate change events seen in ice core records known as Dansgaard-Oescheger (D-O) cycles. Pointing to a North Atlantic origin, D-O cycles only well expressed in Greenland ice core reocrds Source:

23 Another view of climate variability during the last glacial cycle and Heinrich Events. The top two panels give the abundance of N. Pachyderma from two North Atlantic ocean cores (DSDP 609, approx. 50 o N, 45 o W, Vema 23-81, approx. 54 o N, 18 o W) while the bottom panel is the δ 18 O record from the GRIP Summit ice core. The dotted lines on the bottom panel illustrate long-term cooling trends. H1-H6 indicate Heinrich Events while YD indicates the Younger Dryas (YD) event. The age scale is approximate [courtesy of G. Bond, Lamont-Doherty Earth Observatory, Palisades, New York]. The YD, a temporary return to cold conditions following the last glacial maximum, is usually viewed as the last of the D-O cycles.

24 Calcium concentrations (ppb) covering the period ka based on GISP2 ice core data. The sample resolution is approximately 2 years through the Holocene, a mean of 3.48 years within the YD (Younger Dryas) and BA (Bolling/Allerod), and 3-15 years during the OD (Older Dryas) [from Mayewskii et al., 1993, by permission of AAAS]. High calcium and dust concentrations during the YD point to an intensified atmospheric circulation over continental regions and increased aridity.

25 Inferred changes in Artemisia, sea surface temperature and sea ice cover in the North Sea during deglaciation [from Rochon et al., 1998, by permission of Elsevier]. The YD was associated with strong cooling in the North Atlantic.

26 What caused D-O cycles? Most ideas invoke episodes of surface freshening in the North Atlantic that disrupt the thermohaline circulation. The are many different ideas, including the concept of a salt oscillator, binge-purge behavior of the Laurentide ice sheet, and massive discharge of freshwater from glacial-dammed lakes. Other ideas involve more direct climate forcing (e,g., periodic changes in solar irradiance) and then subsequent climate responses to surface freshening.

27 The Laurentide Ice Sheet and the routing of overflow from the Lake Aggasiz basin (dashed line) to the Gulf of Mexico just before the Younger Dryas (a) and routing of overflow from Lake Aggasiz through the Great Lakes to the St. Lawrence and northern North Atlantic during the Younger Dryas (b) [from Broecker et al., 1989, by permission of Nature]. Massive discharge of freshwater into the North Atlantic from the melting Laurentide Ice Sheet could have disrupted the thermohaline circulation, initiating the YD event.

28 Deglaciation

29 Notable Events The Holocene Thermal Maximum (HTM) saw disappearance of the major continental ice sheets. Its onset is linked to perihelion in July and stronger axial tilt, leading to July solar radiation at 65 deg. N being at a maximum positive anomaly at around 9 ka. However, the onset and termination of the HTM varied strongly by region, in part reflecting proximity to the shrinking ice sheets. Late Holocene Cooling: Climatic deterioration after the HTM. The Medieval Warm Period ( AD, North Atlantic Sector) and Little Ice Age (16th and 17 th centuries, but variable). Warming over the past 100 years

30 Variations in the onset and termination of the Holocene Thermal Maximum (HTM) over northern North America (left) and some of the proxy records documenting the event (right) [Source: Kaufman et al., 2004].

31 Records of Northern Hemisphere temperature variations over the last 1300 years. Panels are (top) annual temperature over the instrumental record, (middle) reconstructions using various proxies, (bottom) overlap of all proxy records in middle panel with shading indicating level of agreement between the different reconstructions. The observed temperature record in the bottom panel is shown in black [Source; IPCC-AR-4, Working Group I Report, Figure 6.10]. The Medieval Warm Period from about seems to have had it strongest expressions over the Northern North Atlantic sector. The Little Ice Age is dated anywhere from to The GSIP-II Greenland ice core records put the maximum cooling from

32 Arctic summer temperature anomalies for the past 2000 years based on a variety of proxy sources. The blue line shows a reconstruction of summer Arctic land temperatures over the last 2000 years, based on a composite of 23 proxy records from lake sediments, ice cores, and tree rings relative to reference period. The shaded area represents variability among different Arctic sites. The red line shows the recent Arctic warming based on instrumental observations [adapted from Kaufman et al. 2009].

33 Reconstructed Arctic temperatures over the past 400 years (each panel) along with estimated time series of (a) methane (b) atmospheric carbon dioxide concentration (c) total irradiance (d) sulfate. Asterisks indicate known large volcanic events [from Overpeck et al., 1997, by permission of AAAS]. Solar irradiance seems to have played a significant role in temperature variability up to about the middle of the 20 th century.