Joseph D. Ortiz

Abrupt onset and termination of the African Humid Period: rapid climate responses to gradual insolation forcing

A detailed (ca. 100 yr resolution) and well-dated (18 AMS ^(14)C dates to 23 cal. ka BP) record of latest Pleistocene–Holocene variations in terrigenous (eolian) sediment deposition at ODP Site 658C off Cap Blanc, Mauritania documents very abrupt, large-scale changes in subtropical North African climate. The terrigenous record exhibits a well-defined period of low influx between 14.8 and 5.5 cal. ka BP associated with the African Humid Period, when the Sahara was nearly completely vegetated and supported numerous perennial lakes; an arid interval corresponding to the Younger Dryas Chronozone punctuates this humid period. The African Humid Period has been attributed to a strengthening of the African monsoon due to gradual orbital increases in summer season insolation. However, the onset and termination of this humid period were very abrupt, occurring within decades to centuries. Both transitions occurred when summer season insolation crossed a nearly identical threshold value, which was 4.2% greater than present. These abrupt climate responses to gradual insolation forcing require strongly non-linear feedback processes, and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non-linear climate sensitivity. The African monsoon climate system is thus a low-latitude corollary to the bi-stable behavior of high-latitude deep ocean thermohaline circulation, which is similarly capable of rapid and large-amplitude climate transitions.

Marine Radiocarbon Evidence for the Mechanism of Deglacial Atmospheric CO2 Rise

We reconstructed the radiocarbon activity of intermediate waters in the eastern North Pacific over the past 38,000 years. Radiocarbon activity paralleled that of the atmosphere, except during deglaciation, when intermediate-water values fell by more than 300 per mil. Such a large decrease requires a deglacial injection of very old waters from a deep-ocean carbon reservoir that was previously well isolated from the atmosphere. The timing of intermediate-water radiocarbon depletion closely matches that of atmospheric carbon dioxide rise and effectively traces the redistribution of carbon from the deep ocean to the atmosphere during deglaciation.

Deep-dwelling planktonic foraminifera of the northeastern Pacific Ocean reveal environmental control of oxygen and carbon isotopic disequilibria

Abstract We assess the utility of four species of living planktonic foraminifera as tracers of thermocline and intermediate water masses in the northern Pacific Ocean, based on their water-column distribution and shell isotopic composition. Assuming oxygen isotopic equilibria with the water column, we infer apparent calcification depths. This allows an estimate of apparent carbon isotopic disequilibria. We then relax the assumption of oxygen isotopic equilibrium to examine habitat effects on kinetic disequilibrium processes. In the California Current, left-coiling Neogloboquadrina pachyderma and Globigerina calida calcify in the upper thermocline. Globorotalia scitula resides within the Shallow Salinity Minumum (potential density σ θ = 25.1–26.7), while Globoquadrina hexagona is associated with the deeper, North Pacific Intermediate Water (σ θ = 26.7–26.9). Apparent carbon isotopic disequilibria corrected for oxygen isotopic disequilibria, range from 1.0–1.9‰ in these asymbiotic species. The carbon isotopic disequilibrium can be modeled as a single exponential function of temperature or a logarithmic function of potential food supply. We infer that carbon isotopic disequilibrium increases with metabolic rate, related to temperature and/or food supply. Kinetic processes of oxygen and carbon isotopic disequilibria yields reasonable depth habitats if the slope of the oxygen:carbon isotopic shift is about 0.35, consistent with culture data. Our finding of a link between environment, metabolism, and isotopic disequilibrium observed in oceanic settings suggests the potential to better reconstruct the structure and biological processes of the upper water column from geologic data.

Enhanced marine productivity off western North America during warm climate intervals of the past 52 k.y

Studies of the Santa Barbara Basin off the coast of California have linked changes in its bottom-water oxygen content to millennial-scale climate changes as recorded by the oxygen isotope composition of Greenland ice. Through the use of detailed records from a sediment core collected off the Magdalena Margin of Baja California, Mexico, we demonstrate that this teleconnection predominantly arose from changes in marine productivity, rather than changes in ventilation of the North Pacific, as was originally proposed.

Diffuse spectral reflectance as a proxy for percent carbonate content in North Atlantic sediments

Diffuse reflectance records from Fern Drift in the North Atlantic faithfully record sediment percent carbonate. A high-resolu- tion, reflectance-based age model for these sediments derived from an orbitally tuned age model for western equatorial Atlantic, Ceara Rise sediments was generated by spectral frequency mapping. Power spectra of the Fern Drift record indicate statistically significant sub- Milankovitch cyclicity at 7.6-8.4 and 4.8-6.1 kyr. We infer that these -8 and -5 kyr cycles document a linkage between North and equa- torial Atlantic climate given our ability to correlate these records. These climate cycles influence Atlantic basin carbonate prior to the intensification of Northern Hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets. The presence of these peaks, which could be related to equatorial clipped precession, implies a possible non-linear response to Milankovitch forcing. Abstract. Diffuse reflectance records from Feni Drift in the North Atlantic faithfully record sediment percent carbonate. A high-resolu- tion, reflectance-based age model for these sediments derived from an orbitally tuned age model for western equatorial Atlantic, Ceara Rise sediments was generated by spectral frequency mapping. Power spectra of the Feni Drift record indicate statistically significant sub- Milankovitch cyclicity at 7.6-8.4 and 4.8-6.1 kyr. We infer that these -8 and-5 kyr cycles document a linkage between North and equa- torial Atlantic climate given our ability to correlate these records. These climate cycles influence Atlantic basin carbonate prior to the intensification of Northern Hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets. The presence of these peaks, which could be related to equatorial clipped precession, implies a possible non-linear response to Milankovitch forcing.

Dynamical Response of the Tropical Pacific Ocean to Solar Forcing During the Early Holocene

Sunny and Cool Changes in solar output cause changes in the amount of radiation that Earth receives from the Sun, which in turn can cause climate variations. The effects of solar variations are not uniform over the globe—owing to the complexity of the climate system, larger solar fluxes may produce warming in one area but cooling in another. Marchitto et al. (p. 1378) present a record of Holocene sea surface temperature in the eastern equatorial Pacific Ocean that shows cooling as solar output increased and warming as the Sun dimmed. These temperature changes resulted from dynamical control of El Niño and La Niña episodes by solar radiative forcing of Earths climate. Enhanced solar activity caused the tropical Pacific to cool into a La Niña–like state during the mid-Holocene. We present a high-resolution magnesium/calcium proxy record of Holocene sea surface temperature (SST) from off the west coast of Baja California Sur, Mexico, a region where interannual SST variability is dominated today by the influence of the El Niño–Southern Oscillation (ENSO). Temperatures were lowest during the early to middle Holocene, consistent with documented eastern equatorial Pacific cooling and numerical model simulations of orbital forcing into a La Niña–like state at that time. The early Holocene SSTs were also characterized by millennial-scale fluctuations that correlate with cosmogenic nuclide proxies of solar variability, with inferred solar minima corresponding to El Niño–like (warm) conditions, in apparent agreement with the theoretical “ocean dynamical thermostat” response of ENSO to exogenous radiative forcing.

The California Current of the Last Glacial Maximum: Reconstruction at 42°N based on multiple proxies

Multiple paleoceanographic proxies in a zonal transect across the California Current near 42°N record modern and last glacial maximum (LGM) thermal and nutrient gradients. The offshore thermal gradient, derived from foraminiferal species assemblages and oxygen isotope data, was similar at the LGM to that at present (warmer offshore), but average temperatures were 3.3° ±1.5°C colder. Observed gradients require that the sites remained under the southward flow of the California Current, and thus that the polar front remained north of 42°N during the LGM. Carbon isotopic and foraminiferal flux data suggests enhanced nutrients and productivity of foraminfera in the northern California Current up to 650 km offshore. In contrast, marine organic carbon and coastal diatom burial rates decreased during the LGM. These seemingly contradictory results are reconciled by model simulations of the LGM wind- field, which suggest that wind stress curl at 42°N (and thus open-ocean upwelling) increased, while offshore Ekman transport (and thus coastal upwelling) decreased during the last ice age. The ecosystem of the northern California Current during the LGM approximated that of the modern Gulf of Alaska. Cooling and production in this region was thus driven by stronger open-ocean upwelling and/or southward flow of high-latitude water masses, rather than by coastal upwelling.

COMPARISON OF IMBRIE-KIPP TRANSFER FUNCTION AND MODERN ANALOG TEMPERATURE ESTIMATES USING SEDIMENT TRAP AND CORE TOP FORAMINIFERAL FAUNAS

We evaluate the reliability of statistical estimates of sea surface temperature (SST) derived from planktonic foraminiferal faunas using the modem analog method and the Imbrie-Kipp method. Global core top faunas provide a calibration data set, while modem sediment trap faunas are used for validation. Linear regression of core top predicted SST against atlas SST generated slopes close to one for both methods. However, the Imbrie-Kipp transfer function temperature estimates had an intercept 1.3oC warmer than modem analog estimates and 1.7oC warmer than recorded atlas SST. The RMS error for the core top data set using the modem analog method (1.5oC) was smaller than that of the Imbrie- Kipp method (1.9oC). SST errors for the sediment trap faunas were not statistically different from those of the core top data set, regardless of method. Developing Imbrie-Kipp transfer functions for limited regions reduced the RMS variability but introduced residual structure not present in the global Imbrie- Kipp transfer function. Dissolution simulations with the sediment trap sample which generated the warmest SST residual for both methods suggests that the loss of delicate warm water foraminifera from midlatitude sediments may be the cause of this thermal error. We conclude that (1) the fauna) structure of sediment trap and core top assemblages are similar; (2) both methods estimate SST reliably for modem foraminifera) flux assemblages, but the modem analog method exhibits less bias; and (3) both methods are relatively robust to samples with low communality but sensitive to selective faunal dissolution.

A theoretical study of the kinetics of the boric acid-borate equilibrium in seawater

y . Dissolved boron in seawater occurs mainly in the form of boric acid B OH and borate B OH . While the 34 equilibrium properties of the dissociation of boric acid have been studied in detail, very little work has focused on the kinetics of the boric acid-borate equilibrium in seawater. Here, we present a theoretical study of the relaxation of the w

The spatial distribution and seasonal succession of planktonic foraminifera in the California Current off Oregon, September 1987 – September 1988

Abstract Preliminary results are presented from the September 1987 to September 1988 deployment of the Multitracers Project, a moored sediment trap experiment in the California Current. Three sediment trap moorings were deployed on a transect off Cape Blanco at 130, 280, and 650 km offshore. The seasonal distribution of foraminifers in the California Current at 42°N reflect variations in the temperatures and water masses present at each site. The winter assemblage of foraminifers at the sites 130 and 280 km offshore comprise mainly the subarctic/transitional species: N. pachyderma (right), N. dutertrei, G. bulloides, G. quinqueloba and G. glutinata. From late June to September 1988 at the site 130 km offshore, G. bulloides, G. falconensis, N. pachyderma (left) and the P-D intergrade category record high fluxes, apparently in response to upwelling conditions. The subtropical species O. universa, G. ruber and T. humilis are most important at the site 650 km offshore. The foraminiferal communities present from September 1987 to September 1988 off Cape Blanco seem to be more diverse but less productive than assemblages from the Alaskan Gyre at Station P (45°N, 150°W) as described in the literature.