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The oceans cover over 70% of the Earth's surface and influence

By Corey Simmons,2014-06-23 11:31
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The oceans cover over 70% of the Earth's surface and influence

    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

    been recovered.

    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, such as corals equations are estimated to be ?0.4-0.7ºC. and bivalves.

    Box 1: Generic paleotemperature equation

    181818182T(C)ab(OO)c(OO) carbwatercarbwater

    18OWhere T is temperature, a, b and c empirically derived constants, the carb

    18Ooxygen isotopic composition of the solid carbonate, the oxygen isotopic water

    composition of the water in which the carbonate precipitated. The quadratic fit is

    based on theoretical predictions of the nature of isotopic fractionation at low vs.

    high temperatures. However, linear (c=0) and quadratic (c?0) fits have proven to

    fit experimental data equally well at warm oceanic temperatures, within a precision of . 0.2C

     Figure 2: Mean sea surface temperature (SST) proxy records for the past 2000 years. SST

    reconstructions from the Caribbean and Sargasso Seas are derived from stable oxygen

    isotopes in planktonic foraminifera, whereas the record from the western Equatorial Pacific

    is based on foraminiferal Mg/Ca ratios. To facilitate comparison, each record was

    normalised by its 2000 years (=2kyr) mean value to depict the departure in SST, in ºC. Data

    adapted from Nyberg et al. (2002), Keigwin (1996), and Stott et al. (2004).

     underestimation of water mass temperatures. II(b) Mg/Ca values However, as long as the sediment core

    The elemental ratio of Mg to Ca (Mg/Ca, in position is at least 100m above the calcite mmol/mol) of foraminiferal calcite increases saturation depth (the lysocline), this bias is exponentially with temperature. This minimized.

    relationship is thermodynamic in origin, but This method of paleothermometry is widely substantial deviations between different used and has proven very useful because it foraminifer species are attributed to allows for more accurate estimates of paleo-;18physiological factors (‘vital effects’). These O values (affected by changes in water

    deviations are empirically calibrated with salinity and ice-volume, see above) from the the help of laboratory culturing experiments. oxygen isotope composition of the same

    The accuracy of temperature estimates from calcite, using the oxygen isotope

    Mg/Ca in foraminifera is about 1 to 1.6ºC. paleotemperature equation. Surface ocean Calcite dissolution, which increases with salinity depends primarily on the balance water depth, affects Mg/Ca ratios in between precipitation and evaporation, thus foraminiferal shells by preferential removal strongly reflecting atmospheric and climatic of Mg. This preservation bias leads to an variability.

     allows to separate the SST from the salinity III. Alkenone unsaturation index signal (in case of the foraminiferal Mg/Ca 18Another widely applied proxy, that offers O values), to assess and resolve and carb18sea surface temperature (SST) estimates seasonal SST signals (O of various carb

    independently from those derived from planktonic foraminifer species; alkenone vs. foraminiferal shell chemistry and foraminifer-based proxies) as well as to microfossil assemblage studies, is based on verify the consistency of results between the organic molecules produced by different methods. Ultimately,

    phytoplankton; namely single-celled, marine paleotemperature variability and trends algae called coccolithophores. Only a few reconstructed from marine sediments should species, Emiliania huxleyi and be correlated to other marine proxy records Gephyrocapsa oceanica, have been (such as corals and bivalves) and span the identified to produce long-chain, unsaturated bridge towards terrestrial proxy records methyl and ethyl ketones (alkenones). These (such as lake sediments and ice cores) at alkenones have various chain lengths (C, comparable time resolution. 37

    C, and C) and two or three double bonds. 3839

    The ratio of di- and tri-unsaturated C Temporal Resolution of Marine 37

    alkenones changes linearly with the ambient Sediments

    temperature in which they are produced, as Work on decadal to centennial timescales empirically determined by numerous demands a very tight age control and precise laboratory culture experiments. This ratio correlation between proxy records if robust appears to be stable after incorporation into conclusions are to be made about rapid marine sediments, and thus offers changes or events of short duration. reconstruction of past sea surface Radiocarbon dating is the most common temperatures. The precision (?1 std) of the technique used to date marine sediments, temperature determination is better than usually carried out on biogenic carbonate 0.3ºC. (for example planktonic foraminifera) or

     organic matter. Carbon contained in ocean A global calibration of the alkenone surface waters is older than the carbon in the unsaturation index in surface sediment atmosphere, the so-called reservoir effect. samples to annual mean SST has shown that Therefore, an additional correction needs to seasonality of primary production and be made to convert marine radiocarbon dates variations in growth rate of the algae seem to calendar years to enable adequate to have no strong effect on the linear correlation between various marine records relationship that was established from as well as with terrestrial proxy records. In culture experiments. The standard error of addition, special coring techniques are the SST estimate using the surface sediment required to minimize the disturbance of the calibration is about 1 to 1.5ºC. In some cases, uppermost centimeters of sediment, which the method may be limited due to the lack of are poorly compacted and therefore easily sufficient organic material contained in the lost in the coring process.

    sediments. In recent years, studies of marine sediments

     have extended to higher temporal resolution.

    Temporal resolution of marine sediment Multi-proxy approach

    The use of more than one paleotemperature records (i.e. how much time is contained proxy within one and the same marine within and in what detail) varies greatly due sediment record is recommendable. This to large regional differences in

    sedimentation rates and bioturbation. Broecker, W.S., 2000. Was a change in thermohaline Bioturbation is the mixing of sediments by circulation responsible for the Little Ice Age? PNAS the burrowing activity of organisms that live 97(4), 1339-1342.

    at the seafloor, thus averaging out the

    climatic signals (such as temperature) Burroughs, W.J., 2001. Climate Change A

    originally contained in seasonal and annual Multidisciplinary Approach. Cambridge University deposits. Annual and decadal resolution is Press, Cambridge, UK, 293 pp.

    only acquired in select areas, with rapid,

    continuous deposition and/or limited Fisher, G., and Wefer, G. (Editors), 1999. Use of bioturbation. Rapid deposition is typically Proxies in Paleoceanography Examples from the

    found in highly productive areas, such as South Atlantic. Springer-Verlag, Heidelberg Berlin, upwelling regions off north- and southwest Germany, 735 pp.

    Africa, western South America and the

    Arabian Peninsula, but bioturbation affects High(er) Resolution Proxy Records:

    the record. The restriction of deep-water Keigwin, L. D., 1996. The Little Ice Age and circulation or depletion of oxygen in bottom Medieval warm period in the Sargasso Sea. Science waters (in so-called ‘oxygen minimum 274, 1504-1508.

    zones’) promotes the undisturbed

    sedimentation of particles by minimizing Nyberg, J., Malmgren, B. A., Kuijpers, A., and Winter, bioturbation, resulting in annually laminated A., 2002. A centennial-scale variability of tropical (also called varved) sediments that may North Atlantic surface hydrography during the late contain seasonally distinct features. Holocene. Palaeogeogr. Palaeoclimatol. Palaeoecol. Examples of such high-resolution marine 183, 25-41.

    sediment records have been recovered from

    the Saanich Inlet (British Columbia), Santa Stott, L., Cannariato, K., Thunell, R., Haug, G.H., Barbara Basin (Californian Margin), Koutavas, A., and Lund, S., 2004. Decline of surface Norwegian fjords, and Arabian Sea temperature and salinity in the western tropical Pacific (Pakistan Margin). More typically, Ocean in the Holocene epoch. Nature 431, 56-59. centennial to millennial resolution is found

    in the open marine environment from which Zhao, M., Eglinton, G., Read, G., and Schimmelmann, K’we can gain invaluable information on ) quasi-annual sea A., 2000. An alkenone (U37

    longer-term climate variability, in which the surface temperature record (A.D. 1440 to 1940) using oceans play a crucial and large role. varved sediments from the Santa Barbara Basin.

     Organic Geochemistry 31, 903-917.

     Sources and Selected References

     Bemis, B.E., Spero, H.J., Bijma, J., and Lea, D.W.,

    Jorijntje Henderiks, 1998. Reevaluation of the oxygen isotopic

    Department of Geology and Geochemistry, composition of planktonic foraminifera: Experimental

    Stockholm University results and revised paleotemperature equations.

     Paleoceanography 13(2), 150-160.

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