Editing Oligocene (section)

==Oceans==
The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene/Oligocene boundary,<ref name="Lyle" /> and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in the Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute.

===Effects of oceanic gateways on circulation===
The opening and closing of ocean gateways: the opening of the [[Drake Passage]]; the opening of the [[Tasmanian Passage|Tasmanian Gateway]] and the closing of the [[Tethys Ocean|Tethys]] seaway; along with the final formation of the [[Greenland]]–[[Iceland]]–[[Faroes]] Ridge; played vital parts in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation.<ref name="Proth" />

====Drake Passage====
[[File:Eocene-Paleocene-circumpolar.svg|right|thumb|250px|Eocene-Oligocene circum-Antarctic oceanic changes]]
The Drake Passage is located between [[South America]] and [[Antarctica]]. Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the [[Southern Ocean]] was its connection to South America. As the South American continent moved north, the Drake Passage opened and enabled the formation of the [[Antarctic Circumpolar Current]] (ACC), which would have kept the cold waters of Antarctica circulating around that continent and strengthened the formation of [[Antarctic Bottom Water]] (ABW).<ref name="Proth" /><ref name="Mack" /> With the cold water concentrated around Antarctica, [[sea surface temperature]]s and, consequently, continental temperatures would have dropped. The onset of Antarctic glaciation occurred during the early Oligocene,<ref name="Via"/> and the effect of the Drake Passage opening on this glaciation has been the subject of much research. However, some controversy still exists as to the exact timing of the passage opening, whether it occurred at the start of the Oligocene or nearer the end. Even so, many theories agree that at the Eocene/Oligocene (E/O) boundary, a yet shallow flow existed between South America and Antarctica, permitting the start of an Antarctic Circumpolar Current.<ref name="Katz" />

Stemming from the issue of when the opening of the Drake Passage took place, is the dispute over how great of an influence the opening of the Drake Passage had on the global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation<ref name="Proth" /> and subsequent global cooling, other studies have suggested that the δ<sup>18</sup>O signature is too strong for glaciation to be the main trigger for cooling.<ref name="Katz" /> Through study of Pacific Ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300,000 years,<ref name="Lyle" /> which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.<ref name="Lyle" />

The latest hypothesized time for the opening of the Drake Passage is during the early Miocene.<ref name="Lyle" /> Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true Antarctic Circumpolar Current. If the opening occurred as late as hypothesized, then the Antarctic Circumpolar Current could not have had much of an effect on early Oligocene cooling, as it would not have existed.

The earliest hypothesized time for the opening of the Drake Passage is around 30 Ma.{{r|Lyle}} One of the possible issues with this timing was the continental debris cluttering up the seaway between the two plates in question. This debris, along with what is known as the [[Shackleton fracture zone]], has been shown in a recent study to be fairly young, only about 8 million years old.{{r|Mack}} The study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would have facilitated an earlier onset of the Antarctic Circumpolar Current. There is some evidence that it occurred much earlier, during the early Eocene.<ref name=Lagemaat2021>{{cite journal |last1= van de Lagemaat |first1= S.H.A. |last2= Swart |first2= M.L.A.|display-authors=etal |date= April 2021|title= Subduction initiation in the Scotia Sea region and opening of the Drake Passage: When and why? |journal= Earth-Science Reviews |volume= 215 |pages= 103351 |doi= 10.1016/j.earscirev.2021.103551|bibcode= 2021ESRv..21503551V |s2cid= 233576410 |doi-access= free |hdl= 20.500.11850/472835 |hdl-access= free }}</ref>

==== Opening of the Tasman Gateway ====
The other major oceanic gateway opening during this time was the Tasman, or Tasmanian, depending on the paper, gateway between Australia and Antarctica. The time frame for this opening is less disputed than the Drake Passage and is largely considered to have occurred around 34 Ma. As the gateway widened, the Antarctic Circumpolar Current strengthened.

==== Tethys Seaway closing ====
The [[Tethys Seaway]] was not a gateway, but rather a sea in its own right. Its closing during the Oligocene had significant impact on both ocean circulation and climate. The collisions of the African plate with the European plate and of the Indian subcontinent with the Asian plate, cut off the Tethys Seaway that had provided a low-latitude ocean circulation.<ref name="von" /> The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.<ref name="Allen" />

====Greenland–Iceland–Faroes====
The gradual separation of the clump of continental crust and the deepening of the tectonic ridge in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area.<ref name="Via" /> More information about the evolution of North Atlantic Deep Water will be given a few sections down.

===Ocean cooling===
Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction<ref name="Green" /> and patterns of species migration<ref name="Bose" /> can also be studied to gain insight into ocean conditions. For a while, it was thought that the glaciation of Antarctica may have significantly contributed to the cooling of the ocean, however, recent evidence tends to deny this.<ref name="Mack" /><ref name="Hay" />

===Deep water===
[[File:Reconstruction of Aglaocetus moreni (fossil baleen whale) (Oligocene; Argentina) (32313481396).jpg|thumb|Reconstruction of ''[[Aglaocetus]] moreni'']]
Isotopic evidence suggests that during the early Oligocene, the main source of deep water was the [[North Pacific]] and the [[Southern Ocean]]. As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of the [[North Atlantic]] began to come into play as well. Computer models suggest that once this occurred, a more modern in appearance [[Thermohaline circulation|thermo-haline circulation]] started.<ref name="von" />

Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts.<ref name="Via" />

The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully.<ref name="Mack" /> Regardless of the time at which the opening of the Drake Passage occurred, the effect on the cooling of the Southern Ocean would have been the same.