Dorothy K. Pak

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.

Early Cenozoic benthic foraminiferal isotopes: Species reliability and interspecies correction factors

[1] Oxygen and carbon isotope records are important tools used to reconstruct past ocean and climate conditions, with those of benthic foraminifera providing information on the deep oceans. Reconstructions are complicatedby interspecies isotopic offsets that result from microhabitat preferences (carbonate precipitation in isotopically distinct environments) and vital effects (species-specific metabolic variation in isotopic fractionation). We provide correction factors for early Cenozoic benthic foraminifera commonly used for isotopic measurements (Cibicidoides spp., Nuttallides truempyi, Oridorsalis spp., Stensioina beccariiformis, Hanzawaia ammophila, and Bulimina spp.), showing that most yield reliable isotopic proxies of environmental change. The statistical methods and larger data sets used in this study provide more robust correction factors than do previous studies. Interspecies isotopic offsets appear to have changed through the Cenozoic, either (1) as a result of evolutionary changes or (2) as an artifact of different statistical methods and data set sizes used to determine the offsets in different studies. Regardless of the reason, the assumption that isotopic offsets have remained constant through the Cenozoic has introduced an ∼1 -2°C uncertainty into deep sea paleotemperature calculations. In addition, we compare multiple species isotopic data from a western North Atlantic section that includes the Paleocene-Eocene thermal maximum to determine the most reliable isotopic indicator for this event. We propose that Oridorsalis spp. was the most reliable deepwater isotopic recorder at this location because it was best able to withstand the harsh water conditions that existed at this time; it may be the best recorder at other locations and for other extreme events also.

A FORAMINIFERAL ISOTOPIC PROXY FOR UPPER WATER MASS STRATIFICATION

Modern oceanographic investigations show that surface ocean warming is associated with increased thickness of the mixed layer, deepening of the thermocline, and reduction of upwelling strength. These changes can profoundly affect surface ocean biological productivity. Proxies to measure past changes in upper water mass structure and stability are often poorly constrained. Stable oxygen isotope studies of planktonic foraminifera collected in sediment traps in Santa Barbara Basin, Southern California, demonstrate that the δ18O difference (Δδ18O) between G. bulloides and N. pachyderma (d.) closely monitors changes in the depth of the thermocline and related thickness and stability of the mixed layer. This proxy can be employed in investigations of past changes in sea surface temperature, upwelling and thermocline strength on the California margin, an area of great sensitivity to global change.

Effects of the 1997–98 El Niño on seasonal variations in suspended and sinking particles in the Santa Barbara basin

The relationship between water column processes and sedimentation was investigated using a five year time series of bi-weekly water column measurements and continuous sediment trap collections in the Santa Barbara Basin, California. Conditions during the strong El Nino period of 1997-98 were compared to those during the previous years and the post El Nino period. Suspended particulate concentrations of chlorophyll a (chl a), particulate organic carbon (POC), particulate organic nitrogen (PON) and biogenic silica (bSi) normally underwent a seasonal cycle characterized by high phytoplankton abundance in the spring, dominated by diatoms, followed by lower concentrations of biogenic particles throughout the rest of the year. Maxima in sinking fluxes of POC, PON, bSi and lithogenic silica (lSi) generally occurred during the summer. Prior to the El Nino period, molar ratios of C/N, Si/C and Si/N were all higher in sinking particulate material relative to particulate material suspended in the upper 75 m. Si/N and Si/C ratios were highest in the spring and summer in both surface and sinking pools. During the 1997-98 El Nino, the seasonal evolution of the density structure of surface waters was altered by the presence of a water mass high in temperature and low in salinity. The depression of the thermocline resulted in concen- trations of nitrate, phosphate and dissolved silicon in the upper 75 m becoming lower that those measured in other years. Mean chl a and bSi concentrations integrated from the surface to 75 m were low on an annual basis, but there were no clear changes in the seasonality of suspended particle concentrations. Perhaps unexpectedly, fluxes of POC, PON and lSi at 470 m increased during the El Nino period. Lower C/N ratios and shorter turnover times suggest increases in the export ratios of POC and PON at that time. We hypothesize that the increase in lSi flux, despite the absence of elevated concentrations of lSi in the upper 75 m, resulted from the lateral advection of particles into the region from the increased riverine discharges at the margins of the basin and subsequent scavenging of the small particles by organic material. Decreases in ratios of C/N, Si/C and Si/N in sinking particles that occurred during the El Nino were sustained until the end of the time series in June 1999, and may have resulted from a shift toward a less diatom dominated pool of sinking particles.  2002 Elsevier Science Ltd. All rights reserved.

Sustained deposition of contaminants from the Deepwater Horizon spill

Significance Despite numerous publications reporting the accumulation of petroleum hydrocarbons associated with the Deepwater Horizon spill on the seafloor, the mechanisms of their delivery to the seafloor remain unclear. We demonstrate sedimentation of black carbon derived from the in situ burning of surface oil slicks for about 2 mo following the cessation of burning while other contaminants from the spill, including bioactive barium derived from drilling mud, continued to sediment for at least 5 mo after the well was capped. We also show that the episodic sinking of spill-associated substances was mainly mediated by marine particles, especially diatoms. Together, these data demonstrate delivery mechanisms of contaminants from the spill to benthic ecosystems in the deep Gulf of Mexico. The 2010 Deepwater Horizon oil spill resulted in 1.6–2.6 × 1010 grams of petrocarbon accumulation on the seafloor. Data from a deep sediment trap, deployed 7.4 km SW of the well between August 2010 and October 2011, disclose that the sinking of spill-associated substances, mediated by marine particles, especially phytoplankton, continued at least 5 mo following the capping of the well. In August/September 2010, an exceptionally large diatom bloom sedimentation event coincided with elevated sinking rates of oil-derived hydrocarbons, black carbon, and two key components of drilling mud, barium and olefins. Barium remained in the water column for months and even entered pelagic food webs. Both saturated and polycyclic aromatic hydrocarbon source indicators corroborate a predominant contribution of crude oil to the sinking hydrocarbons. Cosedimentation with diatoms accumulated contaminants that were dispersed in the water column and transported them downward, where they were concentrated into the upper centimeters of the seafloor, potentially leading to sustained impact on benthic ecosystems.

Santa Barbara basin study extends global climate record

A fundamental goal of Earth science is to understand the remarkable instability of late Quaternary global climate prior to the beginning of the Holocene, about 11,000 years ago. This unusual climate behavior was characterized by millennial-scale climate oscillations on suborbital timescales, and a distinctive ‘sawtooth’ pattern of very abrupt glacial and stadial terminations (within decades) followed by more gradual global cooling [e.g., Dansgaard et al., 1993; Hendy and Kennett, 1999]. The fact that both major (glacial) and minor (stadial) cooling periods in Earths climate were terminated by similar abrupt warming episodes suggests a common mechanism driving such rapid changes in global climate. Understanding the causes of this instability is crucial given developing concerns about global warming, yet knowledge about this climate behavior has been essentially confined to the last 150,000 years or so, owing to the absence of available sequences of sufficient age and chronological resolution. The high-resolution paleoclimate record from the Greenland ice cores is limited to about 110 thousand years ago (ka),and although Antarctic ice cores now extend back to more than 740 ka [European Project for Ice Coring in Antarctica, 2004], these latter cores primarily provide information about high-latitude conditions at much lower resolution than is required to address abrupt climate change.

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.

Sea Surface Temperatures in the Western Equatorial Pacific During Marine Isotope Stage 11

Mg/Ca data from ODP Hole 806B on the Ontong Java Plateau, western equatorial Pacific, indicate that marine isotope stage (MIS) 11 was the warmest interglacial episode of the last 450 ky. Sea surface temperatures (SSTs) for MIS 11, as calculated from Mg/Ca, reached just above 30°C. MIS 11 is also characterized by the longest sustained period (-20 ky) of SST ≥ 29°C. The maximum in MIS 11 SST is flanked by strong minima in SST (25 to 26°C) in glacial MIS 10 and 12. The changes in SST, as indicated by Mg/Ca, lead the changes in δ 18 O at the climate transitions by as much as 7-9 ky, suggesting that temperature changed before ice volume and/or local hydrology. Warm tropical Pacific SST during MIS 11 is supported by uplifted reefs of coeval age containing warm-water fauna on the Chilean coast. A super-interglacial in the tropical Pacific during MIS 11 would undoubtedly have influenced global climate through direct heat exchange and influence on atmospheric H 2 O and CO 2 .

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.

The LPTM gas hydrate dissociation hypothesis: New evidence from the western North Atlantic

The late Paleocene thermal maximum (LPTM) c. 55.5 Ma was the most abrupt global warming event of the Cenozoic. Previous work (Dickens et al. 1995) has linked this warming to a massive release of methane from the deep ocean. Here we summarize lithologic, faunal, seismic, and isotopic evidence to support the hypothesis (Katz et al. 1999). ODP Site 1051 on the Blake Nose (subtropical western North Atlantic) contains an exceptional LPTM record (Figs. 1, 2) because the Paleocene section is greatly expanded, contains sediment cycles related to the precessional frequencies (c. 20 k.y. duration), and was deposited on the lower continental slope (c. 2,000 m depth). Thus, this record provides an unprecedented opportunity to study rates and timing of chemical and faunal changes associated with the LPTM. At Site 1051, c. 55% of benthic foraminiferal taxa disappeared in the latest Paleocene within a fraction of a precessional cycle (<5,000 years; Fig. 2). The majority (60%) of these last appearances occurs immediately below the carbon isotope excursion (CIE), where faunal diversity plummets from c. 25–30 to c. 5–10 taxa/sample. Bulimina spp. dominates the surviving fauna, indicating decreased dissolved O2 conditions; at the same time, partly dissolved foraminiferal tests indicate increased ΣCO2 and decreased carbonate ion. A similar rapid, pronounced benthic foraminiferal extinction event (BFEE) and environmental conditions mark the onset of the LPTM at other deep-sea locations and probably resulted from a combination of factors, including increased temperature, decreased dissolved O2, and enhanced carbonate dissolution. Post-extinction re-establishment of benthic assemblages was gradual at Site 1051, with species originations occurring over 200 k.y. Benthic foraminifera (Oridorsalis spp.) δO and δC decrease by c. 1.5‰ and c. 3.0‰, respectively, across a c. 20 cm interval coincident with the BFEE; bulk sediment δC decreases by c. 1.2‰ across the same interval (Fig. 2). Precessional cyclicity in the expanded LPTM section at Site 1051 allows the first direct temporal comparison between theoretical and observed δC changes. Remarkable similarity between modeled and observed records supports the release and oxidation of CH4 in the deep Atlantic during the LPTM. Our results indicate that carbon was input over 5,000 to 7,000 years, with a c. 140 k.y. return to initial conditions as excess carbon was output to the rock cycle. Thermal dissociation and rapid release of 1 to 2 × 10 g of CH4 from marine gas hydrate reservoirs on continental slopes can explain these observations. Methane can escape from gas hydrate reservoirs only through sediment failure. At Site 1051, upper Paleocene benthic foraminifera are consistent with deposition in the lower bathyal zone (1,000–2,000 m), which is the ideal depth on a continental margin to monitor effects of CH4 release. An intraformational mud clast interval immediately below the CIE at Site 1051 provides evidence for slope failure linked to CH4 release (Fig. 2). Matrix and clast sediments are very similar in: 1) major and trace element composition (XRF results); 2) pre-BFEE benthic forami-