Carol J. Pride

The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern North Pacific

The nitrogen isotopic composition of time-series sediment trap samples, dissolved NO~ , and surficial sediments was determined in three regions along the margin of the eastern North Pacific: Monterey Bay, San Pedro Basin, and the Gulf of California (Carmen and Guaymas Basins). Complex physical regimes are present in all three areas, and each is influenced seasonally by coastal upwelling. Nevertheless, sediment trap material evidently records the isotopic composition of new nitrogen sources, since average d15N is generally indistinguishable from d15N values for subsurface NO~ . Surficial sediments are also very similar to the average d15N value of the sediment traps, being within 1&. This di⁄erence in d15N between trap material and sediment is much less than the previously observed 4& di⁄erence for the deep sea. Better organic matter preservation at our margin sites is a likely explanation, which may be due to either low bottom O 2 concentrations or higher organic matter input to the sediments. All sites have d15N for sub-euphotic zone NO~ (8—10&) substantially elevated from the oceanic average (4.5—5&). This isotopic enrichment is a result of denitrification in suboxic subsurface waters (Gulf of California) or northward transport of denitrification influenced water (Monterey Bay and San Pedro Basin). Our results therefore suggest that downcore d15N data, depending on site location, would record the intensity of denitrification and the transport of its isotopic signature along the California margin. Temporal variations in d15N for the sediment traps do appear to respond to upwelling or convective injections of NO~ to surface waters as

Nitrogen isotopic variations in the Gulf of California since the Last Deglaciation: Response to global climate change

High-resolution records of the nitrogen isotopic composition of organic matter (δ15Norg), opal content, and opal accumulation rates from the central Gulf of California reveal large and abrupt variations during deglaciation and gradual Holocene changes coincident with climatic changes recorded in the North Atlantic. Homogenous sediments with relatively low δ15Norg values and low opal content were deposited at the end of the last glacial period, during the Younger-Dryas event, and during the middle to late Holocene. In contrast, laminated sediments deposited in the two deglacial stages are characterized by very high δ15Norg values (>14‰) and opal accumulation rates (29–41 mg cm−2 yr−1). Abrupt shifts in δ15Norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling, vertical mixing, and/or the latitudinal position of the Intertropical Convergence Zone.

Sea-surface temperature anomalies associated with the 1997-1998 El Niño recorded in the oxygen isotope composition of planktonic foraminifera

Anomalously warm sea-surface temperatures and associated high rainfall propagated northward from the eastern equatorial Pacific along the western margin of North America during the 1997–1998 El Nino. We present data from the Guaymas Basin (Gulf of California) and the Santa Barbara Basin (Southern California Borderlands) that clearly demonstrate that the oxygen isotope composition of planktonic foraminifera accurately records the local sea-surface temperature changes related to the El Nino phenomenon. On the basis of this observation, the varved sediments accumulating in these basins should contain a detailed history of both the occurrence and intensity of past El Nino events.

Biogenic silica fluxes and accumulation rates in the Gulf of California

The Gulf of California, though small in size, plays an important role in the global silica cycle. The seasonal pattern of biogenic silica flux in the gulf is closely related to that of phytoplankton biomass levels and is controlled by changes in weather and hydrographic conditions. The highest opal fluxes ([approximately] 0.35 g[center dot]m[sup [minus]2][center dot]d[sup [minus]1]) occur during winter and spring, and they are comparable to those measured in some of the most productive ecosystems of the world. Approximately 15%-25% of the biogenic silica produced in surface waters is preserved in gulf sediments, a figure significantly higher than the average global ocean preservation rate. However, the flux of opal at 500 m water depth is less than 25% of that being produced at the surface, suggesting that most of the recycling of biogenic silica in the Gulf of California occurs in the upper water column. 28 refs., 3 figs.

Varve formation in the Gulf of California: Insights from time series sediment trap sampling and remote sensing

The varved sediments that accumulate in the central Gulf of California (Guaymas and Carmen Basins) provide a record that allows for the resolution of annual to decadal-scale climate variability in this region. Time-series sediment trapping, combined with remotely sensed observations of sea surface temperature (AVHRR) and color (CZCS), have been used to examine the mechanism of varve formation in the central Gulf. SST and surface pigment concentration records for both the mainland and Baja sides of the Gulf display similar seasonal trends. High temperatures and low pigment concentrations occur synchronously on both sides of the central Gulf from June through to October. In association with the 1991/92 El Nino, warm temperatures persist in the central Gulf until December. Low SSTs typically occur from December through May, with high pigment concentrations marking the period from November through April. The summer-early fall (June–October) is a time of high terrigenous and biogenic sediment fluxes. During this time of year, the total flux is dominated by eolian-transported lithogenic material and dark laminae are deposited during this period. During El Nino years, high terrigenous fluxes may extend into winter. Light laminae represent deposition from November through May, during which time biogenic material dominates the total sediment flux. High opal fluxes beginning in November are attributed to seasonal cooling of surface water and upward mixing of nutrients. True upwelling conditions do not develop in the Gulf until late winter-spring. During this time, the total flux is still dominated by opal but the magnitude of the flux is reduced. This may be an artifact of phytoplankton grazing and not lower production rates.