Marine Sediment Proxy Records
Pacific. An El Niño event is reflected by an
anomalous warming of surface ocean waters Why Study Ocean Water Temperature?
The oceans cover over 70% of the Earth’s in the eastern tropical Pacific, lasting three surface and influence climate on a global or more seasons, due to diminished strength scale. Heat exchange between the ocean’s of westward blowing trade winds in surface and the atmosphere is crucial to both connection to shifting atmospheric pressure oceanic and atmospheric circulation patterns. anomalies over Darwin (Australia) and All ocean basins are connected, and ocean Tahiti (South Pacific). This leads to a drastic waters flow from basin to basin in a decrease in fish populations along the coasts continuous, closed fashion driven by of Ecuador and Peru. On neighbouring differences in water temperature and salinity continents it is linked to extreme weather (and hence density). This is often called the patterns such as droughts, heavy rainfall and thermohaline ‘conveyer belt’. Heat is floods. Seasonally, sea surface temperatures released to the atmosphere at high latitudes vary mainly due to changes in the duration in the North and South Atlantic, where and intensity of sunlight (long summer vs. cooling surface waters sink down to form short winter days) as well as storm activity. cold, deep water that flows from the Atlantic
into the Indian and Pacific Oceans. These Hazards of Heating the Oceans
deep flows surface again in the northern Seawater retains heat much better than air, Indian and Pacific Oceans, to travel back to leading to the supposition that most heat the Atlantic as warmer, shallow water related to global warming would be currents. Thermohaline circulation expected to be incorporated into the oceans. influences climate on millennial to multi-Interactions between the atmosphere and the centennial time-scales, for example, changes oceans, and thus global climate patterns, are in the mode and rate of deep water likely to change under the influence of formation have been postulated to explain increased surface ocean temperatures. Ocean colder climate during the Little Ice Age (ca. water temperatures strongly influence the 1700 to 1900 A.D.). abundance and distribution of marine On shorter, interannual to decadal timescales, animals and plants, of crucial importance to global sea surface temperatures are strongly fisheries. Warming of sea surface waters can influenced by atmospheric variability. cause marine ecologies to shift their habitat Prominent climatic phenomena include the and/or to be disrupted from stability. El North Atlantic Oscillation (NAO) and El Niño events are prime examples of such Niño Southern Oscillation (ENSO). The first (regional) disruptions.
refers to shifts in the gradient of atmospheric
sea-level pressure between the How do we reconstruct past seawater Arctic/Icelandic region and the subtropical temperatures?
region near the Azores, which either Reliable seawater temperature estimates are strengthens or weakens the Westerlies crucial to any reconstruction and modeling blowing over the Atlantic. This phenomenon of past ocean salinity and density, water is strongly reflected in European climate column stratification, thermohaline patterns. The Southern Oscillation refers to circulation, and ice volume. A number of the quasi-cyclic behaviour of the interaction proxies are available for this purpose; the between the atmosphere and the equatorial main and most commonly used methods are
Figure 1: 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
MODIS image courtesy of NASA (modis.gsfc.nasa.gov).
discussed in the sections below. Ongoing unicellular zooplankton), and diatoms research will likely provide more detailed (siliceous unicellular phytoplankton). insights to existing proxies, the development The transfer technique is based on grouping of new ones, as well as improved species within an assemblage by Q-mode measurement techniques. factor analysis, which is subsequently
related to SST by multiple linear regression I. Faunal and floral assemblages in the calibration process. An alternative Temperature calculations based on faunal approach, the modern analog technique and floral microfossil assemblages have (MAT), statistically compares fossil been widely applied, using various well-assemblages to modern assemblages that are established statistical techniques and tagged with the overlying modern SST. Both regional or global calibration datasets. This methods are dependent on extensive methodology builds on the fact that the reference data sets that must be calibrated to abundances and geographic distribution of present-day SST. Overall, there seems to be different marine plankton species are tightly little difference in results between the two linked to sea surface temperature (SST), and methods, as long as the fossil group and that the percentages of different species calibration sets are the same. The accuracy contained in a fossil assemblage also reflect of SST estimates varies per calibration, but the sea surface temperatures they originally is typically 1-2ºC. An improvement of the lived in. The main marine microfossil paleotemperature determinations mainly groups studied include planktonic depends on the improvement of the foraminifera (calcareous unicellular reference data set, by adding more core-top zooplankton), radiolaria (siliceous samples and applying uniform taxonomy.
One major limitation is posed by the degree II(a) Stable oxygen isotope composition of preservation of the fossil assemblages The oxygen isotope composition of 18used. Typically, dissolution will foraminiferal calcite (，O) depends on carb
preferentially affect fragile, thinly walled the ambient water temperature at the time of species, which commonly are indicators of calcification. The other most important warmer SST, thereby introducing a bias factor is the isotopic composition of the towards cooler SST reconstructions. seawater, correlated to local salinity, which
varies through time and from place to place.
II. Foraminifer shell chemistry In the longer term (millennial scale), A primary measure of (past) ocean water seawater is affected by whole-ocean shifts in temperatures lies in the chemical analysis of oxygen isotopic composition reflecting calcite shells of marine organisms called storage of lighter oxygen isotopes in foraminifera. Foraminifera are single-celled continental ice sheets. Thus, temperature protists that secrete calcium carbonate shells estimates have to be corrected for seawater around their cells. The chemistry of these composition (see Box 1).
calcite shells provides information about the The reconstruction of temperatures is further chemical and physical conditions in which complicated by a strong linear correlation 18they grew. Two main types of foraminifera between ，O and the carbonate carb
are distinguished, both comprising chemistry (pH and/or carbonate ion numerous different species. Planktonic concentration) of the ambient water as well 18foraminifera live in the upper few hundred as species-specific differences in ，O carb
meters of the water column, whereas (caused by so-called ‘vital effects’), as benthonic foraminifera live on, or within, demonstrated in laboratory cultures of living the seafloor. Foraminifer shells are foraminifera. 18abundantly found as fossils in marine The ratio of the heavy isotope O to the 16sediments and thus serve to reconstruct past lighter O measured in a given carbonate ocean conditions, particularly surface and shell is reported as a deviation from the deep-sea water temperatures. same ratio measured in a standard, in permil Note that the following geochemical (‰). One permil corresponds to an apparent methods are also extensively used in change in temperature of about 5ºC. tracking seawater temperature variability in Standard errors of various paleotemperature other biogenic carbonates