Why are ocean sediment cores, ice cores, and coral reefs so special?Paleoceanography is a young science. If you had been a respectable 'majoritarian' scientist of the year ____ you wouldn't havThe limits of “paleoceanography”Paleoceanographic IndicatorsPaleoceanographic IndicatorsSome simple questions:Paleoceanographic IndicatorsTypical oceanic sedimentation pattern:A Brief Review of Climate Events during the Past 70 Million YearsEvents 1Sea-level estimates from drilling submerged coral terracesClimate variations during the past 150,000 years:Deep sea 18O for the past 350,000 yearsMilankovitch hypothesis: Deep-sea oxygen isotopes compared to summer insolation at 65°NClimate variations during the past 150,000 years:Deep Sea 18O compared to Greenland Ice Core 18OThe past 2 million yearsAbsolute chronology and its importance for paleoclimatologyHow do we estimate a time scale for a marine sediment core or ice core?MIT OpenCourseWare http://ocw.mit.edu 12.740 Paleoceanography Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.Paleoceanographic TracersandMajor Paleoceanographic and Paleoclimate Eventsof the Past 70 Million Years12.740 Lecture 1 Spring 2008Why are ocean sediment cores, ice cores, and coral reefs so special?Although evidence for past climate is found on land as well, thelongest relatively complete record of climate change is contained in deep-sea sediment cores. Although ice cores are short in duration and limited in geographic coverage, they are unparalleled in detail and in their ability to reconstruct past atmospheric composition. With sub-annual resolution, surface reef-building corals contain detailed information about tropical paleoclimate on times scale of decades to millennia.Paleoceanography is a young science. If you had been a respectable 'majoritarian' scientist of the year ____ you wouldn't have believed:• 1850 Massive glaciers once covered the northern continents• 1900 The earth is older than 20-40 m.y.• 1950 There have been more than 4 major advances and retreats of massive continental glaciers in the last 2 m.y.• 1965 Ice advances/retreats are paced by variations in the earth's orbit.The limits of “paleoceanography”Most of the seafloor is <200 m.y. old; precious little is >100 m.y. This time scale sets the practical horizons of paleoceanography as most practitioners know it. Of course, oceans existed before then and some clues to them are preserved in rocks on the continent - but this is "geology" not paleoceanography; the tools and techniques available overlap but are used in very different manner."Typical" oceanic sedimentation rates: 0.1 - 3 cm/103 yrs, so 500,000 years is represented by 0.5 m to 15 m (covered by piston cores); 100 my requires 100m - 3000m (covered by DSDP, especially hydraulic piston coring). Bioturbation of upper 3-15 cm (in most but not all sites) reduces the time resolution obtainable from these sediments and induces artifacts into timing, precision, and magnitude.Paleoceanographic IndicatorsAlthough paleoceanographers would like to be able to specify temperature, salinity, nutrient content, pCO2, etc., we can often only know these indirectly through biological, chemical, and physical properties of sediments. How can we go about reconstructing these properties?Paleoceanographic Indicators• Paleotemperatures (e.g. oxygen isotopes) based on chemical/physical principles. Problems: assumption of equilibrium etc. may not be valid in low temperature and/or biological systems; isotopic composition of seawater must be known or assumed.• Bio-ecological paleotemperatures (e.g. Imbrie-Kipp method) based on modern-day correlations between temperature and the distribution of organisms in the ocean and in sediments. Problems: Correlation is observed, but causation is assumed but not proven (i.e. are population distributions really due to temperature tolerances of organisms?). How reliable is this assumption? Example: polar bears live quite well in the warm San Diego Zoo; the limit on their presence in temperate zones reflects not their temperature tolerances but rather their whiteness which aids their survival in the arctic. While it may be safe to assume that polar bears will not thrive naturally in warm climates, you have to be careful inferring temperature from their distribution. It is reasonable to expect that other factors are important to the distributions of organisms: light and nutrients (floral species); food availability and preferences (faunal species); ecological interactions (both).• Ocean paleochemistry: δ13C; Cd/Ca, Ba/Ca, Zn/Ca (tell us something about the distribution of nutrients and alkalinity in the ocean); carbonate dissolution indicators tell us about carbonate saturation in the deep ocean. Problem: reliability of tracers under certain conditions.• Terrigenous sediment sources: types of minerals; grain size; windblown materials vs. physical transport (water and turbidity currents); erosion.• Geological Time Scale: how do we determine time in geological systems? Most often, ourdirect information is spatial position: depth in ice or sediment core, etc.. Sometimes we have radioactive tracer information - e.g. 14C, 230Th/U - but these may need calibration or have accuracy problems. Most often, we determine time indirectly, via cross correlation. As the primary information on true time expands, the ages of our samples is altered as if it were written on a rubber band.Some simple questions:Paleoceanographic Indicators• The "shifting paleotracer sand" problem: Paleoindicators rarely prove to be as simple as they are initially assumed to be, and sometimes their interpretation changes radically. In part, this arises from the indirect nature of the estimated properties relative to measured properties; in part, it arises out of the limited knowledge available at the time that a tracer is introduced. A certain measure of historical perspective is needed to understand the limits and capabilities of paleo tracers.• "50 Year Event” (etc.) problem: some events occur frequently (on geological time scales) but infrequently on human (or graduate student thesis or research grant) time scales. How do you calibrate something like this.Typical oceanic sedimentation pattern:The study of paleoclimatology in deep sea cores requires stable indicators of past conditions and a reliable time scale. But the seafloor moves vertically