Ingrid L. Hendy

Oxygen minimum zone expansion in the eastern tropical North Pacific during deglaciation

concentrations, lamination preservation and d 15 N values from cores located between 540–750 m water depth in the Gulf of Tehuantepec, Southern Mexico. These results differ from similar proxies found even 5� N of the site, where there is evidence for OMZ changes related to rapid climate change in the Northern Hemisphere. Instead, OMZ expansion in the Gulf was contemporaneous with changes in sea ice extent and zonal wind shifts around Antarctica, pointing to Subantarctic Mode Water and Antarctic Intermediate Water as likely sources of oxygen-depleted water. These observations reinforce the importance of the Southern Ocean as a primary modulator of northern hemisphere ocean climate, as far as 15� N in the ETNP. Citation: Hendy, I. L., and T. F. Pedersen (2006), Oxygen minimum zone expansion in the eastern tropical North Pacific during deglaciation, Geophys. Res. Lett., 33, L20602, doi:10.1029/ 2006GL025975.

Broad region of no sediment in the southwest Pacific Basin

A broad region, nearly the size of the Mediterranean Sea, exists in the central South Pacific Ocean that is devoid of sediment and has been so since the Late Cretaceous. The requirements for remaining sediment free are very low biological productivity, a shallow calcite compensation depth, essentially no dust input, and no deposition of hydrothermal oxides and hydroxides. One or two of these conditions are common, but nowhere else do all four occur. The combined effect of these sediment-inhibiting factors is a consequence of crustal age, seawater chemistry, and atmospheric, oceanographic, and physiographic isolation. Furthermore, this unique combination of conditions has prevailed for more than 80 million years.

Revised ∼2000-year chronostratigraphy of partially varved marine sediment in Santa Barbara Basin, California

Sediment in the deep center of the Santa Barbara Basin (SBB) is almost completely laminated for the portion representing the past ∼2000 years and has been utilized as an archive for high-resolution paleoceanography since the 1970s. Unequivocal proof of the presence of varves in SBB sediment throughout the 20th century has been uncritically used to assume that deeper laminations are varves as well and that they can be counted down-core to arrive at a reliable varve chronology for the past ∼2000 years. The advent of radiocarbon accelerator mass-spectrometric (AMS) dating of sub-milligram-sized organic terrigenous plant fragments and charcoal enabled us to independently date SBB sediment without the underlying uncertainty of variable marine radiocarbon reservoir ages. It was determined that the traditional SBB varve-count ages remain valid from the present down to ∼1700 AD, whereas not all deeper laminations represent varves. Depending on depth, the newly revised chronostratigraphy deviates from the traditional varve count by up to ∼400 years. Here, we present (i) a historic overview of the SBB varve chronology, (ii) a critique of the extended, traditional “varve chronology” and (iii) the rationale behind our new chronology that overcomes the long-standing misunderstanding and bias in lamination counting that was assumed to be “varve counting” below the ∼1700 AD level. Evidence from other California offshore locations indicates that the oxygenation of the deeper water column has been decreasing over the past few hundred years, and this facilitated a transition from laminated and possibly intermittently varved sediment to continuously varved sediment in the SBB.

Redox-controlled preservation of organic matter during "oAE 3" within the Western Interior Seaway

During the Cretaceous, widespread black shale deposition occurred during a series of Oceanic Anoxic Events (OAEs). Multiple processes are known to control the deposition of marine black shales, including changes in primary productivity, organic matter preservation, and dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors. The youngest of these events—the Coniacian to Santonian OAE 3—resulted in a prolonged organic carbon burial event in shallow and restricted marine environments including the Western Interior Seaway. New high-resolution isotope, organic, and trace metal records from the latest Turonian to early Santonian Niobrara Formation are used to characterize the amount and composition of organic matter preserved, as well as the geochemical conditions under which it accumulated. Redox sensitive metals (Mo, Mn, and Re) indicate a gradual drawdown of oxygen leading into the abrupt onset of organic carbon-rich (up to 8%) deposition. High Hydrogen Indices (HI) and organic carbon to total nitrogen ratios (C:N) demonstrate that the elemental composition of preserved marine organic matter is distinct under different redox conditions. Local changes in δ13C indicate that redox-controlled early diagenesis can also significantly alter δ13Corg records. These results demonstrate that the development of anoxia is of primary importance in triggering the prolonged carbon burial in the Niobrara Formation. Sea level reconstructions, δ18O results, and Mo/total organic carbon ratios suggest that stratification and enhanced bottom water restriction caused the drawdown of bottom water oxygen. Increased nutrients from benthic regeneration and/or continental runoff may have sustained primary productivity.

Effects of productivity, glaciation, and ventilation on late Quaternary sedimentary redox and trace element accumulation on the Vancouver Island margin, western Canada

Variations in chalcophile and redox-sensitive trace elements are examined at high-resolution intervals from a ~50 kyr long sediment core (MD02-2496) from the Vancouver Island margin. Enrichments of Ag, Cd, Re, U, and Mo above lithogenous levels, signifying sedimentary suboxia and anoxia, occurred during the early Holocene and Bolling/Allerod, and during warm interstadial events of Marine Isotope Stage (MIS) 3. Down-core trace element profiles co-vary with productivity proxy records (opal, CaCO3, and marine organic carbon), and with sedimentary nitrogen isotope ratios, which reflect variably enriched nitrate upwelled from intermediate waters that were transported northward from the Eastern Tropical North Pacific. The similarity of the MD02-2496 record with records from the southern portion of the California Current System (CCS), and to the Greenland ice core oxygen isotope record during warm climate intervals, suggests that sedimentary redox conditions along the California Current responded to local productivity, to North Atlantic climate change and to tropical Pacific surface water processes via long-distance teleconnections. Concentrations of trace elements and productivity proxies were relatively depleted during the Younger Dryas, cool stadial events of MIS 3, and in two episodes of glaciomarine sedimentation from ~14.7 to 30.5 kyr BP (last glacial maximum, LGM), and from 44 to 50.4 kyr BP. Cordilleran Ice Sheet advancement onto the Vancouver Island continental shelf during the LGM led to intervals of increased terrigenous sedimentation and greatly reduced productivity not seen in the southern portion of the CCS, and along with ventilation of North Pacific Intermediate Waters, resulted in brief sedimentary oxic conditions.

The California Current System as a transmitter of millennial scale climate change on the northeastern Pacific margin from 10 to 50 ka

A high resolution record of δ18O and Mg/Ca-based temperatures spanning 10–50 ka has been reconstructed from the Vancouver margin of the northeastern Pacific Ocean (MD02-2496) from two planktonic foraminiferal species, Neogloboquadrina pachyderma (s.) and Globigerina bulloides. While δ18Ocalcite appears synchronous with Dansgaard-Oeschger Interstadials (DOIs) throughout the record, millennial scale variability in sea surface temperatures (SSTs) and reconstructed δ18Oseawater are frequently out of phase with Greenland climate. Changes in water mass characteristics such as δ18Ocalcite and enriched δ15N events apparently responded to millennial-scale climate change during Marine Isotope Stage 3 (MIS 3), such that negative δ18Ocalcite excursions coincided with heavier δ15N. These water mass characteristic shifts are suggestive of the presence of surface water advected from the Eastern Tropical North Pacific (ETNP) by relative strengthening of the California Undercurrent (CUC) bringing warm, salty tropical waters poleward. The linkage between the strength of the CUC on the NE Pacific margin and millennial-scale climate change may be related to increased sea surface heights off Central America as the Intertropical Convergence Zone (ITCZ) shifted northward in response to changes in North Atlantic Ocean circulation. Poor correlations between proxies exist through late MIS 3 into MIS 2. Ice sheet growth could have disrupted the linkage between CUC and the NE Pacific margin as the Laurentide Ice sheet disrupted atmospheric circulation and the Cordilleran Ice Sheet increasingly influenced regional paleoceanography.

Assessing oxygen depletion in the Northeastern Pacific Ocean during the last deglaciation using I/Ca ratios from multiple benthic foraminiferal species

Paleo-redox proxies are crucial for reconstructing past bottom water oxygen concentration changes brought about by ocean circulation and marine productivity shifts in response to climate forcing. Carbonate I/Ca ratios of multiple benthic foraminifera species from ODP Hole 1017E – a core drilled within the Californian oxygen minimum zone (OMZ), on the continental slope – are employed to re-examine the transition from the well oxygenated last glacial into poorly oxygenated modern conditions. The redox and export productivity history of this site is constrained by numerous proxies, used to assess sensitivity of I/Ca ratios of benthic foraminifera to changes in bottom- and pore water O2 concentrations. Reconstructed iodate (IO3-) availability from the I/Ca ratio of epifaunal (Cibicidoides sp.), shallow infaunal (Uvigerina peregrina), and deep infaunal (Bolivina spissa) foraminifera. The reconstructed IO3- availability profile is used to determine the contribution of bottom water O2 relative to oxidant demand on pore water O2 concentrations. These results suggest that high export productivity on the California Margin drove pore low water O2 concentrations during the Bolling. In contrast low bottom water O2 concentrations at 950 m water depth only contributed to reduced sediments during the Allerod. Increased contribution of modified North Pacific Intermediate Water to the California Current System ventilated the California OMZ during the late glacial and the Younger Dryas such that water overlying the site was oxygenated. These results highlight the promising potential of this new proxy for understanding the relative influence of bottom water O2 concentration and pore water oxidant demand on OMZs.

Ironing out carbon export to the deep ocean

In PNAS, Lopes et al. (1) present a novel approach to quantifying the efficiency of the biological carbon pump as Earth’s climate warmed from the ice bound glacial episode (26,000–18,000 y before present) to the equable climate of the present interglacial (10,000 y BP until present). The biological carbon pump begins with atmospheric CO2 transformation into chemical energy by marine photosynthesizers. The particulate organic carbon (POC) from these organisms is then transported into the deep ocean to be buried in sediments. An efficient biological pump increases carbon burial at the ocean floor and this can impact the Earth’s climate system. Past intervals of high marine carbon burial have been associated with atmospheric greenhouse gas removal leading to planetary cooling.

Climate and Anthropogenic Controls of Coastal Deoxygenation on Interannual to Centennial Timescales

Understanding dissolved oxygen variability in the ocean is limited by the short duration of direct measurements, however sedimentary oxidation-reduction reactions can provide context for modern observations. Here we use bulk sediment redox-sensitive metal enrichment factors (MoEF, ReEF, and UEF) and scanning X-ray fluorescence (XRF) records to examine annual-scale sedimentary oxygen concentrations in the Santa Barbara Basin from the Industrial Revolution (AD ~1850) to present. Enrichments are linked to measured bottom water oxygen concentrations after 1986. We reveal gradual intensification of the coastal oxygen minimum zone (OMZ) on the southern California margin coinciding the 20th century anthropogenic warming trend that leads to reduced oxygen solubility and greater stratification. High-frequency interannual oscillations become more prominent over the last three decades. These are attributed to local ‘flushing events’ triggered by the transition from El Nino to La Nina conditions, which further amplify changes in the extratropical southern Californian OMZ.