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Flux and taxonomic composition of the diatom assemblage was determined for biweekly sediment traps in the central Gulf of California over a 2‐year period. Fluxes and relative abundance of taxa are not significantly different in traps from the eastern side of the Guaymas Basin and the western side of the Carmen Basin, indicating that patterns of diatom production are uniform across the central gulf at all seasons of the year. The annual maximum in diatom flux occurred during the winter season of both years, either as a sustained event over many weeks (1990) or as a series of short blooms (1991–1992). Of the most abundant taxa, only hyalochaete Chaetoceros species showed a clear seasonal cycle, with a flux maximum in the spring of both years. Factor analysis defines five assemblages representing (1) summer and fall conditions of surface stratification and nutrient limitation, (2) winter bloom triggered by onset of strong northwesterly winds, (3) late spring to summer production probably related to sporadic events during periods of weak southeasterly winds, (4) spring 1991 inferred to reflect the distal edge of a coastal upwelling phase, (5) lateral advection of resuspended sediments from the continental shelf, possibly resulting from trapped coastal waves during summer hurricanes and from storms during the 1991–1992 El Niño‐Southern Oscillation (ENSO). The diatom data support the inference that subtropical Pacific water penetrates deeper into the gulf during ENSO events. In order to compare the sediment trap record to a sediment core, a second factor analysis was done using only the taxa which are preserved in the sediment and omitting the most abundant taxa, which showed little downcore variation. This produces five factors representing (1) summer‐fall low production, (2) early winter bloom, (3) late winter‐spring upwelling, (4) ENSO conditions and (5) a late spring‐summer assemblage of uncertain significance. Variation of species characterisic of each factor was examined in piston core Atlantis II 125‐8 56JPC taken from the oxygen minimum zone in Guaymas Basin. Correlation to nearby Deep Sea Drilling Project Hole 480 indicates that the core penetrates to about 15 ka, with laminated sediments in the Holocene (0–10 ka) and late deglacial (11–13 ka) and massive (nonlaminated) sediments around Younger Dryas time (10–11 ka) and before 13 ka. Downcore variations in the diatom assemblage do not correspond directly to presence of laminations, implying that the processes controlling bottom water oxygen content are not determined by conditions of surface water production. Presence of calcareous nannofossils downcore correlates inversely with two diatom assemblages (ENSO and winter bloom indicators) and not with occurrence of laminations, suggesting that occurrence of calcareous nannofossils in the sediment is at least partly a function of surface water production. Prior to 12 ka, production was lower than at present during all seasons, implying weaker winds throughout the year; subtropical waters extended to the midgulf as...

The lithology of deglacial sediments from the Bering Sea includes intervals of laminated or dysaerobic sediments. These intervals are contemporaneous with the occurrence of laminated sediments from the California margin and Gulf of California, which suggests widespread low-oxygen conditions at intermediate depths in the North Pacific Ocean. The cause could be reduced intermediate water ventilation, increased organic carbon flux, or a combination of the two. We infer abrupt decreases of planktonic foraminifer δ 18 O at 14,400 y BP and 11,650 y BP, which may be a combination of both freshening and warming. On the Shirshov Ridge, the abundance of sea-ice diatoms of the genus Nitzschia reach local maxima twice during the deglaciation, the latter of which may be an expression of the Younger Dryas. These findings expand the extent of the expression of deglacial millennial-scale climate events to include the northernmost Pacific.

The relative abundance of diatom taxa in 216 core tops has been quantified and compared, using cluster analysis, with assemblages in six cores located beneath different water masses of the North Pacific Ocean. All but one sequence span at least 1.5 m.y., (early Pleistocene to Holocene), and one extends to 2.6 Ma. The quantitative abundance of each taxon and the space‐time occurrence of each cluster are interpreted as tracers of oceanographic conditions. In addition to high‐frequency (glacial‐interglacial) oscillations, the records display rapid steplike changes at 2.4 Ma, 1.6–1.5 Ma, 1.0 Ma, 0.8–0.7 Ma, and 0.35 Ma. Two secondary transitions, recorded only in the northwestern cores, occur at 2.0 Ma and 1.1 Ma. We conclude that both the high‐frequency oscillations and the stepwise transitions reflect variations in extent of subarctic water masses, under the influence of the Siberian high‐pressure and Aleutian low‐pressure atmospheric cells. We further conclude that (1) the subarctic water mass has developed progressively and attained its modern structure only in the late Pleistocene; (2) some stages of this evolution may have occurred earlier in the eastern subarctic; (3) the Holocene is an atypical “warm” extreme, so that the present is not a very good key to the past; (4) there were several glaciations between 2.4 and 2.0 Ma; and (5) the Subtropical Gyre is more conservative than the Subarctic Gyre, and major changes in the region have been due to forcing from the north. We propose a conceptual model of oscillating metastable climatic modes, maintained by internal feedback; and suggest that steplike changes may occur when one set of boundary conditions is temporarily exceeded.

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