Dyke Andreasen

Regional climate shifts caused by gradual global cooling in the pliocene epoch

The Earths climate has undergone a global transition over the past four million years, from warm conditions with global surface temperatures about 3 °C warmer than today, smaller ice sheets and higher sea levels to the current cooler conditions. Tectonic changes and their influence on ocean heat transport have been suggested as forcing factors for that transition, including the onset of significant Northern Hemisphere glaciation ∼2.75 million years ago, but the ultimate causes for the climatic changes are still under debate. Here we compare climate records from high latitudes, subtropical regions and the tropics, indicating that the onset of large glacial/interglacial cycles did not coincide with a specific climate reorganization event at lower latitudes. The regional differences in the timing of cooling imply that global cooling was a gradual process, rather than the response to a single threshold or episodic event as previously suggested. We also find that high-latitude climate sensitivity to variations in solar heating increased gradually, culminating after cool tropical and subtropical upwelling conditions were established two million years ago. Our results suggest that mean low-latitude climate conditions can significantly influence global climate feedbacks.

Enhanced circulation during a warm period

The Early Pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today. To characterize thermohaline circulation during this warm period (4.4-3.1 Ma) we combined new benthic foraminiferal isotopic data from the southeast Atlantic with published data, and reconstructed Pacific and Atlantic Ocean nutrient distributions. The data indicate enhanced ventilation of the deep Atlantic and intermediate Pacific during the Early Pliocene. Enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of North Pacific and Atlantic subtropical gyres was probably also enhanced. Future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation.

Out of the tropics: the Pacific, Great Basin lakes, and late Pleistocene water cycle in the western United States

Changing Rains The water cycle of the western United States has varied dramatically across the glacial cycles of the Pleistocene, possibly because of changes in the tracks of the storms that deliver moisture to the region. Lyle et al. (p. 1629) present evidence from a collection of Great Basin lakes which show that water levels rose over the last 20,000 years because of moisture transported from the tropical Pacific, not from a southward diversion of the westerly storm track. Furthermore, the timing of the lake level highs in the Great Basin shows a progression from south to north that does not coincide with the northward progression of wet intervals. Precipitation source regions for western North America changed substantially over the last deglaciation. The water cycle in the western United States changed dramatically over glacial cycles. In the past 20,000 years, higher precipitation caused desert lakes to form which have since dried out. Higher glacial precipitation has been hypothesized to result from a southward shift of Pacific winter storm tracks. We compared Pacific Ocean data to lake levels from the interior west and found that Great Basin lake high stands are older than coastal wet periods at the same latitude. Westerly storms were not the source of high precipitation. Instead, air masses from the tropical Pacific were transported northward, bringing more precipitation into the Great Basin when coastal California was still dry. The changing climate during the deglaciation altered precipitation source regions and strongly affected the regional water cycle.

Remote forcing at the Last Glacial Maximum in the tropical Pacific Ocean

We present results of a Last Glacial Maximum (LGM) wind stress sensitivity experiment using a high-resolution ocean general circulation model of the tropical Pacific Ocean. LGM wind stress, used to drive the ocean model, was generated using an atmospheric general circulation model simulation forced by LGM boundary conditions as part of the Paleoclimate Modeling Intercomparison Project (PMIP) [Broccoli, 2000]. LGM wind stress anomalies were large in the western half of the basin, yet there was a significant hydrographic response in the eastern half. This ocean model experiment hind casts changes that are in close agreement with paleoceanographic data from the entire region, even without the explicit modeling of the air-sea interactions. Data and model both predict that the annual average thermocline tilt across the basin was enhanced. Data and model are consistent with a stronger equatorial undercurrent which shoaled to the west of where it does today, and stronger advection of water from the Peru Current into the east equatorial Pacific and across the equator. Paleoproductivity and sea surface temperature (SST) data are interpreted in light of the modeling results, indicating that paleoproductivity changes were related to wind-forced dynamical changes resulting from LGM boundary conditions, while SST changes were related to independent, possibly radiative, forcing. Overall, our results imply that much of the dynamic response of the tropical Pacific during the LGM can be explained by wind field changes resulting from global LGM boundary conditions.

High-precision measurement of phenylalanine δ15N values for environmental samples: a new approach coupling high-pressure liquid chromatography purification and elemental analyzer isotope ratio mass spectrometry.

RATIONALE Compound-specific isotope analysis of individual amino acids (CSI-AA) is a powerful new tool for tracing nitrogen (N) source and transformation in biogeochemical cycles. Specifically, the δ(15)N value of phenylalanine (δ(15)N(Phe)) represents an increasingly used proxy for source δ(15)N signatures, with particular promise for paleoceanographic applications. However, current derivatization/gas chromatography methods require expensive and relatively uncommon instrumentation, and have relatively low precision, making many potential applications impractical. METHODS A new offline approach has been developed for high-precision δ(15)N measurements of amino acids (δ(15)N(AA)), optimized for δ(15)N(Phe) values. Amino acids (AAs) are first purified via high-pressure liquid chromatography (HPLC), using a mixed-phase column and automated fraction collection. The δ(15)N values are determined via offline elemental analyzer-isotope ratio mass spectrometry (EA-IRMS). RESULTS The combined HPLC/EA-IRMS method separated most protein AAs with sufficient resolution to obtain accurate δ(15)N values, despite significant intra-peak isotopic fractionation. For δ(15)N(Phe) values, the precision was ±0.16‰ for standards, 4× better than gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS; ±0.64‰). We also compared a δ(15)N(Phe) paleo-record from a deep-sea bamboo coral from Monterey Bay, CA, USA, using our method versus GC/C/IRMS. The two methods produced equivalent δ(15)N(Phe) values within error; however, the δ(15)N(Phe) values from HPLC/EA-IRMS had approximately twice the precision of GC/C/IRMS (average stdev of 0.27‰ ± 0.14‰ vs 0.60‰ ± 0.20‰, respectively). CONCLUSIONS These results demonstrate that offline HPLC represents a viable alternative to traditional GC/C/IMRS for δ(15)N(AA) measurement. HPLC/EA-IRMS is more precise and widely available, and therefore useful in applications requiring increased precision for data interpretation (e.g. δ(15)N paleoproxies).

A Pliocene to recent history of the Bering Sea at Site U1340A, IODP Expedition 323

Fossil diatoms are the principal component of Bering Sea sediments and reflect the paleoceanographic history of the region. Diatom accumulation rates and relative abundances at International Ocean Discovery Program (IODP) Site U1340A are presented. Overall, the total diatom productivity record from 4.9 Ma to the present day reveals a fourfold reduction at circa 4.2 Ma from ~45 × 107 down to 11 × 107 valves/g (wet sediment), signifying a major shift in the upwelling and/or nutrient regime, coinciding with the end of the late Miocene-early Pliocene bloom identified in the eastern equatorial Pacific and California margin. Further abrupt shifts in the diatom assemblage occur at (1) 2.78–2.55 Ma, (2) 2.0–1.8 Ma, and (3) 1.0–0.88 Ma. (1) At 2.78–2.55 Ma, the appearance of sea ice-related species marks the regional cooling associated with the expansion of Northern Hemisphere ice sheets, subsequent development of stratified, nutrient-depleted waters, and increased influence of Western Basin Water masses (most likely due to the suppressed inflow of the Alaskan Stream). (2) Rapid cooling between 2.0 and 1.8 Ma indicates increased sea ice duration and/or frequency. This, coupled with low sea level stands caused prolonged closure of the Aleutian Passes, coupled with further increased Western Basin Water inflow. (3) The shift to 100 ka glacial/interglacial cycles at the middle-Pleistocene transition (1.0–0.88 Ma) marked an increase in upwelling-related species, indicating enhanced surface water mixing. These records confirm that the development and changing dynamics of sea ice in the Bering Sea played a major role in sub-Arctic Ocean circulation and is an integral component of global climate change.