Eric J. Tappa

Chronology of the pleistocene oxygen isotope record: 0–1.88 m.y. B.P

Abstract Detailed oxygen isotope records from various ocean basins and representing the last 1.88 m.y. are correlated using nannofossil biostratigraphy and paleomagnetic stratigraphy. These correlations establish the global nature of oxygen isotope stages 23 through 63 in the early Pleistocene to latest Pliocene. A composite isotope record for the last 1.88 m.y. reveals that the mid-Pleistocene change in climate regime was a complicated response that lasted from approximately 0.9 to 0.6 Ma and not a simple shift from one climate mode to another. The proposed chronology for the extended isotope stages provides a chronostratigraphic framework for detailed studies of paleoceanographic processes in the early Pleistocene and paves the way for application of oxygen isotope stratigraphy to early Pleistocene deep-sea and continental margin drilled sections.

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

Interlaboratory comparison study of calibration standards for foraminiferal Mg/Ca thermometry

An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752-1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

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.

Sediment fluxes and varve formation in Santa Barbara Basin, offshore California

Results from a year-long sediment-trap study provide the first direct observations of seasonal variability in sediment fluxes to the sea floor in Santa Barbara Basin, offshore California, and are used to evaluate varve formation in this basin. Sediment fluxes throughout the year are dominated by lithogenic material; biogenic sediments (primarily silica) are of secondary importance. We conclude that the combination of high lithogenic fluxes and low biogenic fluxes during the fall-winter period results in the deposition of dark laminae. The onset of upwelling and high surface productivity in the spring results in high biogenic silica fluxes that last into the summer. The relative contribution of silica to the total sediment flux is highest during this spring-summer period, and we suggest that light laminae are formed at that time. This is consistent with previous suggestions that sediment input is the primary factor controlling varve formation in this basin.

Increased marine sediment suspension and fluxes following an earthquake

Earthquakes are commonly cited as one possible triggering mechanism for turbidity flows—dense sediment–water plumes that can transport large volumes of sediment great distances down slope—in both marine and lacustrine settings. Heezen and Ewing were the first to make such a suggestion, attributing breaks in a sea-floor telephone cable in the North Atlantic Ocean to turbidity flows generated by the 1929 Grand Banks earthquake. Anumber of workers have consequently used sedimentary turbidite records to reconstruct the earthquake histories of various regions. Here we present direct observations of a seismically induced turbidity flow. Measurements of light scattering and sediment fluxes in the Cariaco basin indicate that the earthquake that occurred along the coast of northern Venezuela on 9 July 1997 resulted in considerable downslope displacement of sediments—probably >105 tonnes into the deep part of the basin. In such a seismically active region, this mechanism of sediment transport may be responsible for a significant component of the long-term sediment accumulation in the basin. Furthermore, this process may result in the sequestration in deep sea sediments of large amounts of carbon initially deposited at shallow depths.

Oceanographic considerations for the application of the alkenone-based paleotemperature U37K′ index in the Gulf of California

Abstract Alkenone concentrations were determined in sediment trap samples and surficial sediments collected from Guaymas Basin (Gulf of California) to investigate the factors that control the relationship between the U37K′ index and SST. The results from the sediment trap study reveal a positive correlation (r2 = 0.5) between alkenone fluxes and satellite AVHRR sea surface temperature (SST) and a strong correlation (r2 = 0.9) between U37K′ and SST over an annual temperature range of 20°C. Although SST and U37K′ are tightly coupled throughout two upwelling seasons, the relationship deviates from the expression developed by Prahl et al. (1988) at temperatures higher than 26°C. The possible reasons for this deviation are varied but include the observed changes in the coccolithophore species assemblage and the evolution of the thermocline during periods of high SST. The seasonal pattern of alkenone fluxes (higher fluxes during warm periods) observed throughout the 1996 to 1997 study period causes a “bias” in the annually averaged U37K′ record, resulting in temperature estimates that are ∼1°C higher than the annual AVHRR SST average. Furthermore, the U37K′ ratios obtained from core-top sediments indicate an additional ∼2°C “warm bias” in the sedimentary record that may be related to the lateral inputs of alkenones from other regions of the Gulf. These results highlight the importance of oceanographic processes in determining the alkenone temperature signal that is preserved in sediments. After accounting for such effects, U37K′ ratios from a box core were used to reconstruct a SST record extending back to the 18th century for the Guaymas Basin. These results indicate a steady 2°C warming of the surface waters of the Gulf from early 1700s to the 1950s, followed by a rapid apparent 1°C cooling between the 1950s and the 1980s.

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.