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Sediments

provided by D. Gutierrez).

Marine sedimentary records of climate change have mostly proved less than ideal as high-resolution climate records. In the deep oceans, the pattern of accumulation of sediments can be almost linear, consistent and complete, but the rate of sedimentation is so slow that decadal or even centennial-scale resolution is difficult and the record can be further smeared by the activity of benthic microbes, which churn the sediment and further degrade resolution. Deep marine sediments have, however, yielded an excellent global picture of multi-millennial climate variability. At this scale waxing and waning of climate produces layered sequences (see Figure 1) that can now be confidently attributed to regular variations in the pattern of the Earth’s movement around the sun and on its axis. These so called orbital forcing factors, or Milankovitch cycles have been a major driver of glacial advance and retreat over at least the past 2 million years.

In a few places sedimentation rates are high enough (generally > metres per year) and benthic activity is low enough than annual- to interannual-resolved layers (sometimes incorrectly called varves, which are defined as sediment couplets that accumulate annually). These sites are generally located on the eastern sides of ocean basins where equatorwards-flowing, nutrient-rich upwelling currents (Figure 2) feed surface plumes of primary producers (mainly diatoms). A combination of low oxygen solubility in the deep-sourced water currents together with oxygen depletion in the surface waters ensures a benthic oxygen depletion zone (called the oxygen minimum zone, (Figure 3) in which the benthic community is retarded and depositional layering is preserved (Figure 4).

Figure 4: Laminated diatomaceous ooze from ODP sediment core (201-1227B-1H: 10-42 cm bsf) from the Peru continental margin upwelling zone. Sediments here about 16,000 calibrated years old. Notice millimetre to centimetre-scale laminations. Image has been contrast enhanced to highlight the layering. High resolution photograph from GeoTek core scanner.

Distinct sediment layers form in response to changes in the composition and flux of particles to the sediment-water interface. They are preserved either due to very rapid accumulation rates or because of the absence of post-depositional disturbance by physical or biological mechanisms. Both laminated and varved sediments can be used to study ENSO variability, particularly using sediment cores from areas strongly affected by ENSO-forced changes in rainfall or water temperature and ecology.