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OCEAN CIRCULATION MODES OF THE PHANEROZOIC: IMPLICATIONS FOR THE ANTIQUITY OF DEEP-SEA BENTHONIC INVERTEBRATES

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It has been suggested that some modern anchialine cave invertebrates originated from deepsea ancestors of considerable antiquity (>100 million years). An alternative view is that such taxa could not have a long bathyal/abyssal history because of extended periods of anoxia in the oceans during the Mesozoic and Cainozoic, and that consequently their ancestors should be sought among shallow-water fauna. In order to assist in the evaluation of these opposing hypotheses, the Phanerozoic record of oxygen-deficient conditions in the water column is reviewed, with special regard for postulated ''anoxic events'' as a causative mechanism for major changes in biodiversity (e.g., mass extinctions). A key issue is the relative importance, through the Phanerozoic, of two types of deep ocean circulation: Halothermal Circulation (HTC), involving the formation of Warm Saline Deep Water (WSDW) in the tropics, promotes anoxia, while Thermohaline Circulation (THC), characterized by Cold Deep Water (CDW) formed at high latitudes, ventilates the deep ocean. Particular attention is paid to the evidence of the mid-Cretaceous Cenomanian-Turonian Boundary Event. The (apparently) widely accepted view that the deep ocean was mainly anoxic until about 40 million years ago (the time of origin of the modern psychrosphere) is challenged. Evidence from the deep-ocean record of bioturbated sediments suggests that there has been a Cold Deep Water component in ocean circulation for at least the last 90 million years (mid-Cretaceous onwards) and possibly throughout the Phanerozoic. This conclusion has important implications for hypotheses about the antiquity of deep-ocean benthonic invertebrate faunas.

Es wurde vorgeschlagen, dass moderne, anchialine Hohleninvertebraten von Tiefseeformen bedeutenden Alters (>100 Millionen Jahre) abstammen. Eine alternative Ansicht besagt, dass solche Taxa aufgrund ausgedehnter, anoxischer Perioden im Mesozoikum und Kanozoikum keine besonders lange bathyale/abyssale Geschichte haben k onnen. Ihre Vorfahren solten daher bei Flachwasserformen gesucht werden. Um zur Evaluierung dieser gegensatzlichen Hypothesen beizutragen, werden Indizien fur sauerstoffarme Verhaltnisse in der Wassersaule wahrend des Phanerozoikums einer neuen Begutachtung unterzogen, mit bezonderer Berucksichtigung postulierter ''anoxischer Ereignisse'' als kausalem Mechanismus fur grossere Anderungen in der Biodiversitat (z. B. Massen-Aussterben). Ein Schlusselfaktor ist die relative Bedeutung zweier Typen von Tiefsee-Zirkulationen wahrend des Phanerozoikums. Die halothermale Zirkulation (Halothermal Curculation, HTC), die zur Bildung warmen, salinen Tiefenwassers (Warm Saline Deep Water, WSDW) in den Tropen fuhrt, fordert Anoxie, wahrend die thermohaline Zirkulation (Thermohaline Circulation, THC), charakterisiert durch in hohen Breiten gebildetes, kaltes Tiefenwasser (Cold Deep Water, CDW) die Tiefsee ventiliert. Besonderes Augenmerk wird auf die Ereignisse an der Cenoman-Turon-Grenze (Mittlere Kreide) gelegt. Die (offensichtlich) weithin akzeptierte Ansicht, dass die Tiefsee bis vor 40 Millionen Jahren grossteils anoxisch war (der Ursprungszeitpunkt der modernen Psychrosphare), wird in Frage gestellt. Hinweise auf durch Bioturbation gestorte TiefseeSedimente lassen vermuten, dass eine kalte Tiefenwasser-Komponente bei den Meereszirkulationen seit mindestens 90 Millionen Jahren (seit der mittleren Kreide), und moglicherweise wahrend des gesamten Phanerozoikums, existiert hat. Diese Schlussfolgerung hat wichtige Auswirkungen auf Hypothesen uber das Alter der benthischen Tiefsee-Evertebratenfauna.

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/content/journals/10.1163/156854099503906
1999-11-01
2015-06-03

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