Can the oceans's temperature a few thousand years back be estimated? Yes it can be done. Here is how..

¹⁸O₈ is 12.5% heavier than ¹⁶O₈, hence is slower in its reactions.  During evaporation of  water the  water with ¹⁶O₈ get into air a little faster than     ¹⁸O₈, over a long period of time as in the ocean, the residual water will be richer in ¹⁸O₈ than it was originally.  So seawater has a little more ¹⁸O₈ than fresh water. Fresh water is  the evaporated portions of sea water.  More impotantly the ratio of the two depend on the temperature.  For every one degree rise in temperature of the ocean the ratio of ¹⁸O₈ to ¹⁶O₈ increases by 0.02 %

The fossil seashells are chiefly made up of CaCO₃. The ¹⁸O₈/¹⁶O₈ ratio in those shells reflect the ratio of water from which they derived the oxygen and that in turn, is a measure of the ocean temperature of that time. Through Carbon dating studies it is possible to estimate the age of the fossil seashells, thereby estimating the temperature of the ocean thousands of years back. Harold C. Urey at Univ. of Chicago made such measurements.

Source: Asimov on Chemistry