Late Cenozoic Arctic Ocean sea ice and terrestrial paleoclimate L. David Carter U.S. Geological Survey, Anchorage, Alaska 99508-4667 Julie Brigham-Grette University of Alberta, Edmonton, Alberta T6G 2E3, Canada Louie Marincovich, Jr., Victoria L. Pease, John W. Hillhouse U.S. Geological Survey, Menlo Park, California 94025 ABSTRACT Sea otter remains found in deposits of two marine transgressions (Bigbendian and Fishcreekian) of the Alaskan Arctic Coastal Plain which occurred between 2.4 and 3 Ma suggest that during these two events the southern limit of seasonal sea ice was at least 1600 km farther north than at present in Alaskan waters. Perennial sea ice must have been severely restricted or absent, and winters were warmer than at present during these two sea-level highstands. Paleomagnetic, faunal, and palynological data indicate that the later transgression (Fishcreekian) occurred during the early part of the Matuyama Reversed- Polarity Chron. Amino acid diagenesis in fossil mollusks suggests that since the later transgression the effective diagenetic temperature (EDT) in the deposits has been about -16 °C, which is about 7 °C colder than modern values and slightly colder than the EDT calculated for the past 125 ka. Such a low EDT suggests that permafrost and perennial sea ice have been present nearly continuously since this transgression. Permafrost probably was absent, however, during the earlier (Bigbendian) transgression. Permafrost and extensive perennial sea ice may have been initiated during the late stages of climatic cooling that spanned the Gauss Normal-Matuyama Reversed-Polarity Chron boundary and led into the first major late Cenozoic glaciation of the Northern Hemisphere. TABLE 1. Alle:lie RATIOS FOR FOSSIL HIATELLA ARCTICA Alle/Ile Transgression Ocean Point Fish Creek Skull Cliff Fishcreekian 0.086 ± 0.004 (6) 0.090 ± 0.018 (11) Bigbendian 0.136 ± 0.014 (12) 0.150 ± 0.025 (8) Note: Ratios for total (free plus peptide bound) acid hydrolysate. Numbers In parentheses Indicate number of samples analyzed. Figure 1. Map showing location of sites dis- cussed in text, and mod- ern limits of perennial and seasonal sea ice. Sea-ice boundaries represent po- sitions for which there is a 50% probability of 5/10 ice cover on September 15 (perennial sea ice) and March 1 (seasonal sea ice) (LaBelle et al., 1983). INTRODUCTION The importance of Arctic Ocean sea ice in amplifying or moderating global changes in climate during the Cenozoic is well known (Clark, 1982). There is no general agreement, however, on the time of initiation of Arctic Ocean sea ice, or whether once it was initiated, there were subsequent ice-free periods. We present marine vertebrate and invertebrate fau- nal evidence that sea ice in the Arctic Ocean was much more limited than today during two marine transgressions of the Alaskan Arctic Coastal Plain that occurred between 2.4 and 3 Ma, and show by paleomagnetic and palyno- logical data that the later high sea-level event occurred during the early part of the Matuyama Reversed-Polarity Chron. Calculations based on measurements of amino acid diagenesis in- dicate that the period since this later transgres- sion was dominated by climates significantly colder than the present one, and we discuss the implications of this for sea ice and permafrost. MARINE TRANSGRESSIONS The two transgressions are the second and third of the six to possibly eight that are repre- sented by sediments of the late Cenozoic Gubik Formation, and have been informally named the Bigbendian and Fishcreekian transgressions (Carter et al., 1986). They are correlated and differentiated across northern Alaska by com- paring the extent of epimerization of L- isoleucine (lie) to D-alloisoleucine (Alle) in fossil mollusks (Brigham, 1985) (Table 1). Fossiliferous sediments of the Bigbendian and Fishcreekian transgressions occur in super- position at Skull Cliff (Fig. 1), where they have been informally named the Killi Creek (equals Bigbendian) and Tuapaktushak (equals Fish- creekian) members of the Gubik Formation (Brigham, 1985). Fossiliferous Bigbendian sed- iments are also well exposed near Ocean Point (Fig. 1) and are described by Nelson and Car- ter (1985). At Fish Creek (Fig. 1), fossiliferous tidal channel sediments of the Fishcreekian transgression are 8 to 9 m thick (Fig. 2, unit 2). These are underlain by estuarine deposits that Carter and Galloway (1985) believed formed during the Fishcreekian transgression, but that Brouwers in Repenning et al. (1986) believed are of significantly greater age. Relative sea level during the Bigbendian transgression was between 35 and 60 m above present mean sea level. Sediments of this trans- gression have been recognized at altitudes of about 35 m, and geomorphic evidence suggests that the maximum altitude reached by any late Cenozoic transgression was less than 60 m (Carter and Galloway, 1985). The tidal channel GEOLOGY, v. 14, p. 675-678, August 1986 675