Sandbox edit
This is a test. This is only a test. What am I testing? The lovely, yet incomplete article below.
Ocean edit
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[1], 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 (or lack thereof) is still a matter of some dispute.
Effects of Oceanic Gateways on Circulation edit
The opening and closing of ocean gateways (the Drake Passage, the Tasmanian Gateway, the Tethys seaway, and the Greenland-Iceland-Faroes sill) played a vital part in reshaping oceanic currents during the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation[2].
Drake Passage edit
The Drake Passage is located between South America and Antarctica. Once the Tasmanian Gateway (between Australia and Antarctica) opened, all that kept the Southern Ocean from being completely isolated was the Drake Passage. The opening of the Drake Passage enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept cold, Antarctic waters circulating about the continent and strengthened the formation of Antarctic Bottom Water (ABW).[3][2] With the cold water concentrated around Antarctica, sea surface temperatures and consequently, continental temperatures would have dropped. The onset of Antarctic glaciation (Oi-1) occurred during the early Oligocene[4], 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 formation of a preliminary ACC. [5]
Stemming from the DOP timing issue is dispute over the extent of the DPO’s influence on global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation,[2], and subsequent global cooling, other studies have suggested that the δO18 signature is too strong for glaciation to be the main trigger for cooling[5]. 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 ka[1], which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling[1].
Late Oligocene Drake Passage Opening edit
The latest-hypothesized time for the Drake Passage Opening (DPO) is during the early Miocene[1]. 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 ACC. If the DPO occurred as late as hypothesized, then the ACC could not have had much of an effect on early Oligocene cooling, as it wouldn't have existed.
Early-Oligocene Drake Passage Opening edit
The earliest-hypothesized time for the DPO is around 30 Ma[1]. One of the possible issues with this timing was the continental debris, as it were, 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 Ma[3]. The aforementioned study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would facilitate an earlier onset of the ACC, but the rapidity of the
Currently, an early Oligocene DPO is favored.
Tasman Gateway Opening edit
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 ACC strengthened.
Tethys Seaway Closing edit
Though the Tethys was not a gateway, but rather a sea in its own right, its closing during the Oligocene had significant impacts on both ocean circulation and climate. The collision of the African plate with the European plate and of the Indian subcontinent with the Asian plate all conspired to cut off the Tethys seaway that had provided a zonal low-latitude ocean circulation[6]. The closure of Tethys built some new mountains (the Zagros range) and drew down more CO2 from the atmosphere, contributing to global cooling[7].
Greenland-Iceland-Faroes edit
The gradual separation of the clump of continental crust and the deepening of tectonic sill in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area[4]. More information about the evolution of North Atlantic Deep Water will be given a few sections down.
Ocean Cooling edit
Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Extinction patterns[8] and species migration[9] can also be studied to gain insight into ocean conditions. For a while, it was thought that the Oi-1 event may have significantly contributed to the cooling of the ocean; however, recent evidence tends to deny this[10][3].
Deep Water edit
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 (GIR) sill deepened, connecting the Norwegian-Greenland sea with the Atlantic Ocean, North Atlantic Deep Water began to come into play as well. Model runs suggest that once this occurred, a more modern-looking thermohaline circulation initialized[6].
North Atlantic Deep Water edit
Evidence for the early Oligocene onset of North Atlantic Deep Water (NADW) lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts[4]
South Ocean Deep Water edit
South Ocean Deep Water (SODW) began in earnest once the Tasmanian Gateway and the Drake Passage opened fully[3]. Regardless of the time at which the DOP occurred, the effect of cooling the Southern Ocean and leading to increased deep-water formation would have been the same.
Notes edit
- ^ a b c d e Lyle, Mitchell (April 2008). "Pacific Ocean and Cenozoic evolution of climate". Reviews of Geophysics. 46 (2): 1–47. doi:10.1029/2005RG000190.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ a b c Prothero, D. (May 2005). "Tertiary to Present | Oligocene". Encyclopedia of Geology: 472–478. doi:10.1016/B0-12-369396-9/00056-3. ISBN 9780123693969.
{{cite journal}}: CS1 maint: date and year (link) - ^ a b c d Mackensen, Andreas (Dec 2004). "Changing Southern Ocean palaeocirculation and effects on global climate". Antarctic Science. 16 (4): 369–389. doi:10.1017/S0954102004002202.
{{cite journal}}: CS1 maint: date and year (link) - ^ a b c Via, Rachael (June 2006). "Evolution of Antarctic thermohaline circulation: Early Oligocene onset of deep-water production in the North Atlantic". Geology. 34 (6): 441–444. doi:10.1130/G22545.1.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ a b Katz, M (May 2011). "Impact of Antarctic Circumpolar Current development on late Paleogene ocean structure". Science. 332 (6033): 1076–1079. doi:10.1126/science.1202122. PMID 21617074.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ a b von der Heydt, Anna (May 2008). "The effect of gateways on ocean circulation patterns in the Cenozoic". Global and Planetary Changes. 1-2. 62 (1–2): 132–146. doi:10.1016/j.gloplacha.2007.11.006.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ Allen, Mark (July 2008). "Arabia-Eurasia cooling and the forcing of mid-Cenozoic global cooling". Palaeogeology, Palaeoclimatology, Palaeoecology. 1-2. 265: 52–58. doi:10.1016/j.palaeo.2008.04.021.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ Green, William (2011). ""Does extinction wield an axe or pruning shears? How interactions between phylogeny and ecology affect patterns of extinction". Paleobiology. 37 (1): 72–91. doi:10.1666/09078.1.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Bosellini, Francesca (February 2008). "Estimating Mediterranean Oligocene-Miocene sea surface temperatures: An approach based on coral taxonomic richness". Palaeogeography, Palaeoclimatology, Palaeobiology. 1-2. 258 (1–2): 71–88. doi:10.1016/j.palaeo.2007.10.028.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link) - ^ Hay, William (September 2004). "Is initiation of glaciation on Antarctica related to a change in the structure of the ocean?". Global and Planetary Change. 1-3. 45: 1–11.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: date and year (link)