Timing of late Cenozoic tidewater glaciation in the far North Pacific

Abstract

The onset of late Cenozoic glacial events in the far North Pacific Ocean is recorded by ice-rafted debris in the Yakataga Formation in the Gulf of Alaska. The dating of these events is controversial. Ages based on molluscs suggest that initial late Cenozoic tidewater glaciation occurred in the early middle Miocene (15-16 Ma). Previous work on planktic foraminifera indicates that this event is no older than late Miocene, probably latest Miocene (5-6 Ma). Resolution of this problem is important because the Yakataga Formation is the thickest, most complete, and best exposed repository of late Cenozoic glaciomarine rocks in the northern hemisphere. Investigation of this record has broader implications for global temperature gradients, paleoceanographic development of the far North Pacific, and northern hemisphere glacial history. New planktic foraminiferal data from onshore and offshore Yakataga Formation sections in the Gulf of Alaska are compared to a regional depositional and chronostratigraphic framework in order to test whether the Yakataga record is anomalous or consistent with the paleoclimatic record of the North Pacific. This comparison reconfirms that initial glaciomarine rocks of the Yakataga Formation are no older than late Miocene. New paleomagnetic data suggest that the base of the Yakataga Formation is within the lower Gilbert polarity chron, consistent with the planktic foraminiferal dating and K/Ar dates on glauconite. Correlation of the Yakataga record to key offshore sections in the far North Pacific (Deep Sea Drilling Project [DSDP] Sites 178, 183, 192) provides additional insights into the paleoclimatic significance of these rocks. Uplift of the Alaska coastal ranges, a necessary prelude to alpine glaciation, occurred during late Miocene time. Diatom biofacies (DSDP Sites 183, 192) indicate warm middle Miocene conditions, climatic deterioration during the late middle and early late Miocene time, and initial tidewater glaciation between 5.0 and 6.7 Ma. A relatively warm mid-Pliocene interval succeeded this initial glaciation, which was, in turn, followed by hemisphere-scale glaciation beginning at ∼2.5 Ma. This second phase of Yakataga glaciation may have begun as early as 3.0-3.5 Ma. Glaciomarine rocks in the basal Yakataga Formation mark an important climatic event in the North Pacific Ocean. They are latest Miocene in age, consistent with regional climatic information, and not a middle Miocene climatic anomaly.