ELSEVIER Palaeogeography, Palaeoclimatology, Palaeoecology 152 (1999) 339–364 Quaternary glacial and deglacial Ostracoda in the thermocline of the Little Bahama Bank (NW Atlantic): palaeoceanographic implications Julio Rodriguez-Lazaro a,Ł , Thomas M. Cronin b a Paleontologı ´a, Universidad del Pais Vasco=E.H.U., Apartado 644, Bilbao, 48080, Spain b U.S. Geological Survey, 955 National Center, Reston, VA22092, USA Received 10 August 1998; revised version received 15 January 1999; accepted 20 January 1999 Abstract We determined faunal and oceanographic changes during the last glacial and deglacial in the Providence Channel, Little Bahama Bank (LBB), using modern ocean (from LBB, Florida–Hatteras Slope and Blake Plateau, western North Atlantic) and late Quaternary (LBB) distributions of the benthic ostracode genus Krithe from the mid-depth (300–1600 m) subtropical North Atlantic Ocean. Nine species of Krithe are limited in their bathymetric distribution by warm bottom water temperatures (or a temperature-related parameter) in the thermocline of the modern Atlantic. During the last glacial interval in the northwest Providence Channel of the Little Bahama Bank, five species of Krithe (K. aequabilis, K. dolichodeira, K. gr. minima, K. reversa and K. trinidadensis) migrated upslope; conversely, during the deglaciation, most Krithe species migrated downslope, re-occupying their deeper niches. These vertical species migrations are attributed to decreased glacial bottom water temperatures and perhaps increased dissolved oxygen during the last glacial and warmer water temperatures during the deglacial. Based upon thermal values of recent depth ranges of selected species of Krithe, we estimate that glacial waters cooled about 4ºC (shallower than 900 m) and about 2ºC (deeper than 900 m) and deglacial waters warmed about the same values in shallow and mid-depth water masses, comparing to modern temperatures. The discovery of common Halocypris, a mesopelagic ostracode, in Little Bahama Bank glacial and deglacial sediments also suggests greater oxygenation relative to the late Holocene.  1999 Elsevier Science B.V. All rights reserved. Keywords: Ostracoda; North Atlantic; thermocline; Quaternary; migrations; palaeoceanography 1. Introduction An important but poorly understood component of late Quaternary climate changes involves changes in mid-depth ocean circulation. It has been proposed from increased benthic foraminiferal δ 13 C values and reduced Cd=Ca ratios during glacial periods that mid-depth oceanic circulation during the last Ł Corresponding author. Fax: C34 94 464 8500; E-mail: gpprolaj@lg.ehu.es glacial period differed from that of the Holocene due to changes in the strength of thermohaline circulation and high-latitude deep-water formation (Boyle and Keigwin, 1987; Boyle, 1988; Curry et al., 1988; Oppo and Fairbanks, 1990). de Menocal et al. (1992) and Oppo and Lehman (1995) also found similar patterns of δ 13 C over longer timescales in the North Atlantic Ocean. Slowey and Curry (1987, 1992, 1995) used stable isotopes of foraminifers to study the structure of the glacial thermocline and intermediate-depth waters in the Providence Chan- 0031-0182/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0031-0182(99)00048-6