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Today I learnt in college that marine sediments can be used to reconstruct the climate up to 500 million years back. I find that number very strange because in another class we learnt that the ocean floor "recycles" every few million years as a result of plate tectonics, and that the oldest ocean floor is 200 million years old.

I asked the lecturer about this, but he said he did not know. So I was hoping anyone here could tell me how many years old the climate we can recover through marine sediment cores is and why.

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  • $\begingroup$ Could it mean that marine sediment core deposits that have been uplifted on land can be used in such a way? The fact that ocean floor changes completely every few 100 million years makes sense due to continental drift and subduction. $\endgroup$
    – Trevor J. Smith
    Jan 14 '19 at 18:12
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The oldest ocean floor in the ocean is no more than 200 million years old. Ocean floor sometimes subducts into the Earth but other times gets lifted to eventually become solid land. Whale fossils have been found in the Andes, deep sea creature fossils in the Alps, and perhaps most famously, bizarre sea life fossils from the mid-Cambrian in the Burgess Shale in the Canadian Rockies.

Fossils embedded in what was sea floor, along with the sediment around those fossils, provide information regarding paleoclimate. One key set of paleoclimate proxies are various isotope and chemical ratios in fossilized seashells. There are lots of fossilized seashells, spanning in age from fairly recent geological past to 500 million years ago.

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The oldest in situ marine sediments recovered by IODP (International Ocean Drilling Program; formerly ODP and DSDP) was (from memory) Late Triassic (so ca. 200 to 230 Ma) and it is as old as in situ marine sediments go, since indeed, as the marine crust gets older, it gets denser and thus is more prone to be subducted. However we do have older marine sediments, that were lifted during various orogenies and that were somewhat metamorphosed but are still viable for paleoclimatic studies (to some reduced extent of course). These lifted and metamorphosed sediments (along with the crust below them and even the upper mantle dragged along) are referred to as ophiolites, and they indeed date as far back as the earliest Phanerozoic (so 500 Ma). As most of the climatic information we are getting from them depends on the planktonic or benthic eukaryotic activity at the time of sedimentation, we cannot really go further back with these.

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  • $\begingroup$ This is a somewhat summarized answer that would need citations and whatnots but I m currently on an expedition and have limited internet for the next month, I'm afraid. $\endgroup$
    – plannapus
    Jan 18 '19 at 22:54
  • $\begingroup$ Ophiolites are igneous rather than sedimentary or metamorphic. $\endgroup$
    – David Hammen
    Jan 18 '19 at 22:58
  • $\begingroup$ Ophiolites can contain the full sequence from marine sediments to crust to upper mantle. I was here referring only to the sedimentary part. $\endgroup$
    – plannapus
    Jan 18 '19 at 23:06
  • $\begingroup$ This should not be the accepted answer. Ophiolites offer very little, if any, information regarding paleoclimate. It's sedimentary rocks, and more importantly the fossils embedded in those sedimentary rocks, that provide that information. $\endgroup$
    – David Hammen
    Jan 20 '19 at 21:31

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