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IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 36 TABLE 2.1 Time Units for the Late Ordovician (Ashgill) and Earliest Silurian Period Age Stage Zone Silurian Llandivery Rhuddanian acuminatus Ordovician Ashgill persculptus Hirnantian extraordinarius Rawtheyan pacificus bohemicus uniformicus complexus mirus Cautleyan typicus complanatus Pusgillian Deglaciation started in the graptolite Glyptograptus persculptus zone. At least part of that zone is coeval with the latter part of the Hirnantian. The Ordovician-Silurian boundary is at the base of the Parakidograptus acuminatus zone. The glaciation was thus entirely within the latter part of the Ashgill. THE PALEOGEOGRAPHIC FRAMEWORK The paleogeographic maps in Figures 2.1 and 2.2 were supplied by C. R. Scotese. The Ashgill and Early Silurian (Llandovery) paleogeography (i.e., the positions of the lands, shelf seas and open oceans of the time) has been developed from remnant magnetism data, glacially derived sedimentary materials and features, and the locales of carbonates and evaporites. The carbonates most likely formed in tropical shallow marine environments. Ocean water masses and ocean currents are proposed based on paleogeography and consideration of modern oceanic circulation patterns. Three primary features stand out (see Figure 2.1 and 2.2): (1) no record of land or shallow marine shelf environments in the Northern Hemisphere north of the tropics; (2) a significant number of plates bearing shallow marine environments aligned essentially east-west within the tropics; and (3) a large land mass, Gondwanaland, that covered the South Pole, with a portion of it extending northward into the equatorial region. The positions of the plates bearing lands and shallow shelf seas, as well as the zoogeographic relationships of the faunas present in the rocks formed in these environments, allow the major features of ocean circulation to be proposed. Late Ordovician-Early Silurian ocean circulation (see Figures 2.1 and 2.2) is based on the assumption that similar insolation patterns existed in the Ashgill as at present. The presence of a large part of Gondwanaland in southern temperate and tropical latitudes suggests that ocean surface circulation near it was influenced by seasonal monsoons. The positions of pressure systems over land and nearby seas would have shifted seasonally, leading to seasonal reversals in surface ocean circulation, as is seen today in the Indian Ocean. Accordingly, surface ocean circulation patterns are proposed for winter and summer seasons (Figures 2.1 and 2.2). Ocean surface circulation north of the Northern Hemisphere tropics would have been zonal. Surface circulation in the tropics would have been influenced greatly by the several plates bearing shallow marine environments. The relatively long north-south orientation of the Laurentian (North America), Baltoscania, and Gondwanaland plates would have resulted in major oceanic circulation that was essentially within a single ocean. The ocean bordered by Gondwanaland and Baltoscania (see Figures 2.1 and 2.2) would have had a unique surface circulation that included a relatively cold polar western boundary current that flowed northward on the west side of Gondwanaland. Upwelling would have been strong and continuous there as a result of Coriolis deflection. The modern analogue of that current is the Humboldt Current off Peru-Chile. A similar but weaker western boundary current probably flowed along the west side of Baltoscania. Pre-Hirnantian Ashgill and Hirnantian brachiopod zoogeography provides clues to potential changes in surface circulation in the ocean between Baltoscania and Gondwanaland. Sheehan and Coorough (1990) recognized four unique zoogeographic faunas, prior to the Hirnantian, along the Baltoscanian-Gondwanaland coasts. A single fauna,
IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 37 Figure 2.1 Paleogeographic maps for Ashgill time showing positions of lands, shelf seas, and open oceans. Ocean surface circulation is suggested. The size of Gondwanaland suggests that monsoonal conditions near it generated seasonal differences in ocean surface circulation. As sea level fell during the latter part of the Ashgill, large areas of the shelves were exposed and ocean surface currents may have been enhanced along shelf margins the Hirnantian fauna, replaced three of the four provincial faunas during the Hirnantian Stage. This zoogeographic change suggests that the current flowing from the tropics south along Baltoscania strengthened to such an extent that it had an influence on the Gondwanaland shelf. The Gulf Stream may be a somewhat analogous modern current. The southerly flowing current would have flowed from the tropics into high latitudes, carrying warm surface water into a cold air regime. When the Gulf Stream originated to carry warm water northward in the past 1.5 to 2 m.y., the infusion of warm water led to enhanced evaporation in a cold air regime and so was a factor in the development of glacial ice. Potentially, the ocean surface current circulation implied by the wide geographic spread
IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 38 of the Hirnantia fauna functioned as the Gulf Stream did during the onset of North Polar glaciation in the Pleistocene. Figure 2.2 Paleogeographic maps for the Llandovery showing positions of lands, shallow shelf seas, and open oceans. Ocean surface water circulation currents are indicated. The size of Gondwanaland suggests that monsoonal conditions were generated near it. Seasonal monsoons would lead to seasonal differences in surface circulation. The shelf seas were sites of marine transgression that commenced in the latest Ordovician and continued into the early part of the Llandovery. Return of stable shelf sea environments during the Llandovery was followed by significant reradiation among marine communities, both benthic and planktic. Sheehan (1982) described development of Llandovery brachiopod communities in shelf sea environments that had become stable following deglaciation and transgression. Base map provided by C. R. Scotese. Glaciation during the Hirnantian Stage resulted in lower sea-level. Surface water current impinged on only the outer margins of plates, and shelf seas were significantly narrower than prior to glaciation. Surface currents may have swept along shelf margins more strongly that when waters were widespread across the shelves. Currents may have slowed when sea-level rose as a result of deglaciation.
