08 - PALEOCEANOGRAPHY How has the Earth's ocean changed...

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Unformatted text preview: PALEOCEANOGRAPHY How has the Earth's ocean changed through time and how do we know? Heat exchange between ocean and atmosphere control oceanic and atmospheric circulation patterns. Reliable past seawater temperature estimates are crucial to any reconstruction / modeling of past ocean salinity, density, stratification, thermohaline circulation, and ice volume. Need paleoceanography proxies... proxies... records of what the ocean was like Scientific deep sea drilling IODP- Integrated Ocean Drilling Program Why the ocean floor? Unmatched length and continuity of records The oceans are a very important component of the climate system with a relatively long response time They integrate climate change over useful timescales (months to millennia) Foraminifera tests ~ calcite (CaCO3) C CO2 Isotopes: 13C/12C Notation: 13C () ( 18O/16O 18O () ( Deep Sea Sediments: "Archives of Earth History" History" Microfossil shell chemistry provides information on ocean temperature & carbon chemistry Ocean temp "proxies" of past ocean conditions Faunal and floral assemblages: biostratigraphy biostratigraphy geographical distribution of different marine plankton species are linked to specific temps zooplankton= foraminifera, radolarians (animals) phytoplankton = diatoms, coccoliths Global sea surface temperatures magnetostratigraphy magnetostratigraphy chemostratigraphy chemostratigraphy (photosynthesizing algae) Need multiple datasets to make robust conclusions Global sea surface temperature as measured by moderate-resolution imaging spectroradiometer (MODIS; one month composite of May 2001). Star symbols indicate locations from where marine sediments with interannual to centennial resolution have been recovered (modis.gsfc.nasa.gov). 1 plankton- drifting organisms that inhabit the pelagic planktonzone (sunlit surface waters) of the oceans. Planktonic microfossils 1 2 plant calcareous 1) coccolith 3) diatoms animal 2) foraminifera 4) radiolarian 3 4 siliceous Biostratigraphy focuses on correlating and assigning relative ages of sediments / sedimentary rocks by using the fossils contained within them Established zonations exist for a large number of groups Many groups also represent specific environments / ocean depths Steinnesbeck et al., 2004 Magnetostratigraphy Using changes in the Earth's magnetic field, Earth' recorded in sediments, to tie to seafloor (basalt) time scales Very valuable during many (but not all) intervals of geological time Uneven chron spacing means that precision of method varies Many questions remain regarding the calibration of absolute ages Rob Coe, UCSC 2 Ocean temp "proxies" (continued...) (continued... Chemostratigraphy Foraminifera shell chemistry single-celled protists that secrete calcium singlecarbonate shells around their cells Global sea surface temperatures chemistry of shells reflects chemistry of water from which they grew 2 types: planktonic forams- living in upper few 100 m forams benthic forams- living on or within the seafloor forams- Global sea surface temperature as measured by moderate-resolution imaging spectroradiometer (MODIS; one month composite of May 2001). Star symbols indicate locations from where marine sediments with interannual to centennial resolution have been recovered (modis.gsfc.nasa.gov). Ocean temp "proxies" (continued...) (continued... Stable oxygen isotope composition Experiments on real biological systems show: The 18O shell/water relationship The oxygen isotope composition of calcite formed in the ocean (18O) depends on the water temperature at the time of formation, and the isotopic composition of the seawater. 18O 16O Two common isotopes of oxygen on Earth The ratio of the heavy isotope 18O to the lighter 16O measured in a given shell is reported as a deviation from the same ratio measured in a standard (permil ). One permil corresponds to an apparent change in temperature of about 5C. High light cultures Low light cultures Bemis et al (1998) Living planktonic Foraminifera Benthic vs planktonic foram records Orbulina universa O. universa Pre-sphere Globigerinoides ruber Globorotalia menardii Symbiotic Non-symbiotic Symbiotic Non-symbiotic Globigerina bulloides Howie Spero Hastigerina pelagica 3 Long term climate change Long term record = average global temperature Composite record from >40 ocean drill sites benthic foraminifera Earth ice-free before ~32 million yrs ago ice Major climate events apparently related to major tectonic events (ex: ice cap around Antarctica with opening of ocean basin) and biological evolution ice complicates record!! Zachos et al., 2001 Ocean water 18O 16 lighter oxygen (16O) 18O = -45 18O = 0 18 more heavy oxygen (18O) Cooke and Rohling, 2001 Ocean 18O and glaciations Redistribution of 16O into glacial ice raises the 18O of seawater Major factor changing whole ocean isotope signature: global glaciations 4 Last 3.5 million years Pleistocene Pliocene The Pliocene (5.4 - 2.4 million yrs ago) The Pliocene world was warmer than at present. The ancient distribution of warm-climate ocean plankton, warmplankton, and of animal and plant fossils on the land, shows that mean annual temperatures in the mid-latitudes were often midseveral degrees higher. The greatest warming seems to have been in the Arctic and cool temperate latitudes of the Northern Hemisphere, where temperatures were often warm enough to allow species of animals and plants to exist hundreds of kms N of the ranges of their nearest present-day relatives. presentBecause there was less ice volume near the poles, sea levels may have been as much as 30m higher than at present during the warmest intervals. The causes of the generally warmer climates of the Pliocene are something of a mystery... may have been related to mystery... changes in ocean circulation patterns, perhaps combined with higher concentrations of greenhouse gases in the atmosphere. Zoom in Zoom in Raymo (1994) Last 800,000 years Imbrie and others (1984) Glaciers, Chamonix, France Ice ages Today's ice caps grew to 3x their current area and were up to 3 km thick km thick Cold weather zones expanded and warm weather zones contracted The land deformed isostatically upon loading by the ice sheets. In some deformed upon loading by the ice sheets. In some locations, the land is still responding isostatically to the removal of the ice. E.g. Scandinavia (1m / century), Ohio valley. Because of isostatic deformation, the land buckled near the edge of the ice. Glacial meltwater pooled at margins of glaciers forming large meltwater meltwater lakes. The Great Lakes are stranded remnants of such! At the climax of the last glaciation (18,000 years ago) accumulation of all of last glaciation (18,000 accumulation of that ice and snow - sea level dropped 120 m. that ice and snow - sea level dropped 120 m. Some of the major geographic differences from the modern Earth (besides the presence of so much continental ice): Beringia - the land bridge across the Bering straites land bridge across the Bering straites Australia, Tasmania, and New Guinea were one large land mass Many of the islands of Southeast Asia were part of the mainland. Last remnant of North American Ice sheets melted ~10,000 years ago. At the end of the last ice age lots of North American animals went extinct especially large plant-eating mammals and the predators that fed on them, including: Four species of elephants (e.g. North American imperial mammoth) horses, sloths, camels, sabre-toothed cat, dire wolf, lions, tigers, bears Boiler up! 5 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue.

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