The California Current, Devils Hole, and Pleistocene climate.

Herbert et al. (1) are to be commended for their convincing and important correlation of midto late-Pleistocene California Current sea surface temperatures (SSTs), California coastal vegetation, and the paleotemperature record in vein calcite at Devils Hole, Nevada. Their intriguing notion linking California Current SSTs and the Laurentide ice sheet, however, is open to question, as is their assertion that the work resolves some pending inconsistencies with the orbital theory of Pleistocene climate. Herbert et al. (1) noted a trend of SST warming in the California Current 10 to 15 thousand years (ky) before each of the past five deglaciations. They considered that early warming to be a regional signal caused by “collapse” of the California Current triggered by growth of the Laurentide ice sheet. Noting a strong linear correlation (r 5 0.7 to 0.78) of the Devils Hole oxygen isotope (]O) time series with their SSTs, Herbert et al. (1) concluded that the early, pre-deglaciation warming exhibited by this continental record must also reflect a regional, not a global, signal. There are several reasons to question their regional-signal thesis. SST warming 5 to 15 ky before the last deglaciation occurred at a minimum of 13 locations in both hemispheres of the Pacific and Atlantic Oceans [figure 2 in (2)]. Levine et al. (3) documented similar early SST warming, before the penultimate deglaciation, at 15 locations in the Indian, Pacific, and Atlantic Oceans. Such widespread early SST warming, at latitudes ranging from 40°N to 44°S, might be attributed to regional or local causes that fortuitously occurred before the deglaciations; however, a global explanation for such behavior appears much more plausible and should be sought. In addition, the atmospheric circulation model for the last glacial maximum (4) cited by Herbert et al. (1) to explain the collapse of the California Current is not supported by sedimentological studies; Muhs and Zarate have noted that “in North America, including the midcontinent, the northwestern United States, and Alaska, paleowinds derived from eolian sediments do not agree with climate model simulations” (5). And although they highlighted a strong linear correlation between their SSTs and the Devils Hole time series, Herbert et al. (1) failed to note the strong correlation of Devils Hole with another well-studied paleotemperature record, the Vostok, Antarctica, ice core, 114 degrees of latitude away. These linear correlations range from r 5 0.61 to r 5 0.92, depending on which of the several Vostok chronologies is utilized (6, 7). If Devils Hole is a regional signal responding, like California Current SSTs, to growth of the Laurentide ice, why is it strongly correlated with Vostok? All of these arguments, taken together, call into question the Laurentide ice sheet–based explanation offered by Herbert et al. for the collapse of the California Current and the ensuing early SST warming off the coasts of southwest Oregon and California. Believing that the early warming at Devils Hole before deglaciation had a regional rather than a global cause, Herbert et al. (1) went on to assert that the Devils Hole record thus “does not pose a fundamental challenge to the orbital (‘Milankovitch’) theory of the Ice Ages.” In making that assertion, however, they failed to inform their readers that the Devils Hole study they cited (8) actually raised not one but five challenges to the orbital theory, three of which have credence solely by virtue of the robust and replicated (9, 10) thermal ionization mass spectrometry (TIMS) uranium-series dating of the Devils Hole ]O time series. The five challenges posed in that study (8) included (i) the early warming before deglaciations, already discussed; (ii) various studies of uranium series– dated corals that indicated that sea level was at or near modern levels as early as 132 to 135 thousand years ago (ka), a finding not easily reconcilable with insolation forcing of the penultimate deglaciation; (iii) the socalled stage 11 problem—that is, the occurrence of a prominent deglaciation and subsequent glaciation in the absence of significant variations in high-latitude summer insolation during the period from ;450 to ;350 ka; (iv) interglacial periods recorded at Devils Hole that are twice as long as prescribed by the orbitally tuned SPECMAP global ice volume record; and (v) the observation that glacial-interglacial cycles become aperiodic and of increasing duration as the record approaches the present day (8).