Reconstructing past changes in the oceans

Today will focus on the main methods used to reconstruct past changes in ocean circulation. A key point to consider is the timescales of ocean processes. The oceans have large response time, the surface ocean taking from months to years and deep oceans taking decadal to centennial time scales (Mackay et al., 2003). Given the range of timescales involved in processes of ocean heat transfer, historic records are thus too short to provide records of ocean system prior to anthropogenic intervention, a goal crucial if we are to place our understanding of past ocean circulation into the context of today’s climate (the role of paleoceanography) (Mackay et al., 2003; Oldfield, 2005).

What do Paleoceaographers use to reconstruct past changes in oceans?

Proxies

-Descriptions for ‘target parameters’ such as past ocean temperatures and salinity, to reveal a greater insight into the past ocean and bordering continents (Mackay et al., 2003; 192).

- The proxies are calibrated in some many to provide a quantification of changes in the target parameters (Mackay et al., 2003).

Marine surface sediments

-          Reconstruct changes in ocean as well as surrounding continents (Mackay et al., 2003). Can reconstruct a range of parameters using marine sediments (e.g. SST, surface and deep circulation patterns).  Also act as storage of information concerning continents (e.g. ice volume (Mackay et al., 2003).

-          Two components; biogenic and lithogenic (Mackay et al., 2003). Former originates in surface water but can receive contribution from ocean bottom water, consists of organic matter (e.g. pollen grains), calcium carbonate (from organisation such as coccolithophores and planktonic foraminifera (Mackay et al., 2003).  The latter is composed of clays, but larger material such as boulders can be deposited, such as iceberg melt. The origin can be found on continents where rock and soil are eroded, and deposited via rivers or icebergs for example, in the ocean.

Corals

-          Provide second major source of evidence for past changes in sea surface temperature (SST) and sea surface salinity (SSS) (Oldfield, 2005).

(Note: with corals and marine sediment, estimates are determined from assemblages of marine organisms such as foraminifera, calculated to modern species distribution)

Stable Isotope ratios (Δ ¹⁸O)

-           As ice forms, sequesters a higher proportion of the lighter isotope (Δ ¹⁶O), which becomes depleted in seawater.  During glacial intervals, the Δ ¹⁸ O values in marine organisms increases, with the converse occurring during interglacials (Mackay et al., 2003)

-          Can be inferred from bubbles in ice cores, relating to changes in ice volume and global sea-level BUT.. links with sea are complex (Mackay et al., 2003)

Paleoceanographers use a range of proxies in order to obtain target parameters to gain a more detailed insight into past oceans and adjacent continents (Figure 1) (Mackay et al., 2003).

Figure 1: Proxies and the corresponding target parameters used by Paleoceanographers

Source: (Mackay et al., 2003).

Are there any issues?

Resolution (Marine sediments)

-          Temporal:  Deep- Ocean sedimentation rates are approximately 2-5 cm/kyr, with productive areas producing a maximum of 20 cm/kyr . Thus, temporal resolution is thus limited to 200 yr/cm (Mackay et al., 2003). Normal marine sediments, active benthic community can mix approximately the top 20 cm of sediment via bioturbation, reducing resolution to an estimated 1000 years/ cm, leaving an estimated 10 data points for the Holocene (Mackay et al., 2003).

-       Spatial:  Marine sediments may contain localized environment and climate information, in comparison to high-resolution records such as ice cores, which offer global climatic information (Mackay et al., 2003).

Recommended Read: Maslin, M., Pike, J., Stickley, C. and  Ettwein, V. (2003) 'Evidence of Holocene Climate Variability in Marine Sediments' in Mackay, A, Batterbee, R., Birks, J. and Oldfield, F. (eds) (2003) Global change in the Holcene, Hodder, London, 185-209.