Diatoms in the Gulf of California: Seasonal flux patterns and the sediment record for the last 15,000 years

Abstract

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...

Late Cretaceous seasonal palaeoclimatology and diatom palaeoecology from laminated sediments

“…The terrigenous flux is interpreted to represent run-off and possible aeolian contribution derived from summer storms driven by a palaeo-North American monsoon (Davies et al, 2012). This is similar to the climate and flux cycle in the modern Gulf of California (Douglas et al, 2007) where abundant large diatom vegetative cells are sedimented in late summer/ early winter in the fall dump (Sancetta, 1995;Kemp et al, 2000). In both locations, the position of Hemiaulus spp.…”

“…(C. centralis, C. perforatus, C. granii, C. oculus-iridis and C. wailesii) are important components of the early winter and winter flux . Similarly, Sancetta (1995) noted high fluxes of Coscinodiscus during the late autumn in the Gulf of California. High flux of large Coscinodiscus spp.…”

“…The closest extant relative of Azpeitiopsis is the genus Azpeitia, with which Azpeitiopsis shares many morphological similarities, a species of which, Azpetia nodulifera, is particularly important in palaeoceanographic studies in the Gulf of California. A. nodulifera is generally rare in coastal settings, but is common in subtropical/tropical gyres (Fryxell et al, 1986;Schrader et al, 1993) and its presence in the Gulf of California can be correlated with the warmer, more oceanic conditions of El Niño events (Sancetta, 1995;Barron et al, 2004). Sediment traps deployed at the Namibia upwelling mooring site (central Benguela System) reveal Azpeitia spp.…”

“…A further adaptation, like that of Hemiaulus is symbiosis with the nitrogenfixer Richelia intracellularis in diatom-diazotroph associations (Villareal and Carpenter, 1989). Extensive sediment trap studies show major fluxes of rhizosolenid diatoms in the late summer to early winter period (Takahashi et al, 1989;Sancetta et al, 1991;Sancetta, 1992;Sancetta, 1995;Lange et al, 2000) and it is regarded as a key player in the fall dump flux when stratification breaks down at the end of the summer . On the other hand rhizosolenid species with Richelia-type endosymbionts formed part of the "Summer export pulse" at the ALOHA site together with Hemiaulus hauckii (Scharek et al, 1999) demonstrating a propensity for summer as well as fall export.…”

“…Valves are robust and heavily silicified, consistent with an ecological preference to high energy environments, and as such are often preserved in sediments and common in the geological record. In the Gulf of California, Sancetta (1995) regards this genus as a shelf dwelling form that is advected into the pelagic realm during summer storms. After valves have been advected into the plankton, they may be transported hundreds of miles offshore (e.g.…”