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Scholarly Works (26 results)

  • Article
  • Peer Reviewed
UC Santa Barbara Previously Published Works (2010)
Variations in the eccentricity (100,000 yr), obliquity (41,000 yr) and precession (23,000 yr) of Earths orbit have been linked to glacial-interglacial climate cycles. It is generally thought that the 100,000-yr glacial cycles of the past 800,000 yr are a result of orbital eccentricity. However, the eccentricity cycle produces negligible 100-kyr power in seasonal or mean annual insolation, although it does modulate the amplitude of the precession cycle. Alternatively, it has been suggested that the recent glacial cycles are driven purely by the obliquity cycle. Here I use statistical analyses of insolation and the climate of the past five million years to characterize the link between eccentricity and the 100,000-yr glacial cycles. Using cross-wavelet phase analysis, I show that the relative phase of eccentricity and glacial cycles has been stable since 1.2 Myr ago, supporting the hypothesis that 100,000-yr glacial cycles are paced by eccentricity. However, I find that the time-dependent 100,000-yr power of eccentricity has been anticorrelated with that of climate since 5 Myr ago, with strong eccentricity forcing associated with weaker power in the 100,000-yr glacial cycle. I propose that the anticorrelation arises from the strong precession forcing associated with strong eccentricity forcing, which disrupts the internal climate feedbacks that drive the 100,000-yr glacial cycle. This supports the hypothesis that internally driven climate feedbacks are the source of the 100,000-yr climate variations. © 2010 Macmillan Publishers Limited.
    Cover page: Links between eccentricity forcing and the 100,000-year glacial cycle
    • Thesis
    • Peer Reviewed
    UC Santa Barbara Electronic Theses and Dissertations (2020)

    Climate states of warm and cold periods have varied on approximately 100,000 year cycles for the last million years. These climate states are represented in similar patterns of global sea level observed in five to seven individual records of sea level over 800-kyr. These records were combined into a 800-kyr long global stack using principal components analysis. A record of δ18O of benthic foraminiferal calcite shows a correlation of 0.9 to the sea level stack (PC1), suggesting a strong sea level influence in the calcite, but a 2-kyr lag with respect to the calcite record suggests that deep ocean temperature precedes the sea level response. Sea level change is estimated to account for nearly 45% of the 100-kyr power of benthic δ18O.

    The principal component analysis also captured regional variation in the sea level records in PC2 and PC3. Regional variations in δ18O of seawater during the modern/Holocene and Last Glacial Maximum (LGM), respectively, may help us to understand ocean and atmospheric circulation associated with these extreme climate changes. A simple three-box model of the Atlantic Ocean was compared to surface and deep gradients in δ18O of seawater from paleoclimate proxy measurements. First, parameters were tuned to realistic modern values that matched modern δ18O of seawater observations. The estimated LGM parameters did not fit the proxy evidence for a surface gradient change of -0.04 per mil or a vertical gradient change of 0.36 per mil. However, the error in the proxy surface and vertical gradient change estimates is quite large (+/-0.29 per mil and +/-0.14 per mil, respectively). An improved fit to the data was achieved by slowing the overturning circulation while increasing Arctic runoff to a modern value. The results of this study suggest that additional LGM δ18O of seawater measurements are needed to constrain ocean and atmospheric circulation changes.

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    Cover page: Glacial-Interglacial changes in sea level and seawater delta-O-18 gradients
    • Article
    • Peer Reviewed
    UC Santa Barbara Previously Published Works (2007)
    We describe the evolution of climate system dynamics by examining the climate response to changes in obliquity and precession over the last 5.3 Myr. In particular, we examine changes in the shape of glacial cycles and the power of obliquity and precession response in benthic δ18O. When the exponential trend in δ18O variance is removed, its spectral power exhibits strong, proportional responses to amplitude modulations in orbital forcing over most of the Plio-Pleistocene. Precession responses correlate with modulations in forcing for the last 5 Myr, but 41-kyr response is sensitive to obliquity modulation only before 1.4 Myr. Where responses are sensitive to modulations in forcing, we demonstrate that glacial cycles are orbitally forced rather than being self-sustained or paced by orbital changes. The shapes of glacial cycles have several nonlinear properties, which may be indicative of glacial-interglacial differences in climate sensitivity or response time. The "saw-tooth" asymmetry of glacial cycles first appears shortly after the onset of major northern hemisphere glaciation, and the relative duration of interglacial stages decreases at 1.4 Myr. Collectively, trends in the shape of glacial cycles and the sensitivity of δ18O to obliquity and precession are suggestive of major transitions in climate dynamics at approximately 2.5 and 1.4 Myr but show no significant change associated with the appearance of strong 100-kyr cycles during the mid-Pleistocene transition. © 2006 Elsevier Ltd. All rights reserved.
      Cover page: Plio–Pleistocene climate evolution: trends and transitions in glacial cycle dynamics
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2013)
      Constraints on the timing of Atlantic meridional overturning circulation (AMOC) changes during the last deglaciation are fundamental to understanding the climate's rapid response to insolation forcing. However, uncertainty about high-latitude North Atlantic (HLNA) radiocarbon reservoir ages has previously precluded robust age model development for this critical region. HLNA reservoir ages also serve as a proxy for AMOC strength. We present regionally averaged HLNA reservoir ages for 0 to 41 thousand years before the present (kyr BP) based on over 500 radiocarbon dates from 33 North Atlantic cores. An early deglacial increase to >1000 14C yr reservoir ages between 18.5 and 16.5 kyr BP suggests reduced AMOC before peak Heinrich Stadial 1 (HS1) ice-rafted debris (IRD). A rapid decrease in reservoir ages coincident with the IRD maximum at 16 kyr BP indicates strong stratification of the upper water column caused by massive freshwater release. © 2013. American Geophysical Union. All Rights Reserved.
        Cover page: North Atlantic circulation and reservoir age changes over the past 41,000 years
        • Article
        • Peer Reviewed
        UC Santa Barbara Previously Published Works (2014)
        ©2014. American Geophysical Union. All Rights Reserved. Benthic δ18O changes are often assumed to be globally synchronous, but studies comparing 2-9 radiocarbon-dated records over the most recent deglaciation (Termination 1) have proposed differences in the timing of benthic δ18O change between the Atlantic and Pacific, intermediate and deep, and North and South Atlantic. Because of the relatively small number of records used in these previous studies, it has remained unclear whether these differences are local or regional in scale. Here we present seven regional benthic δ18O stacks for 0-40 kyr B.P. that include 252 records with independent regional age models constrained by 852 planktonic foraminiferal 14C dates from 61 of these cores. We find a 4000 year difference between the earliest termination onset in the intermediate South Atlantic at 18.5 (95% confidence interval: 17.9-19.0) kyr B.P. and the latest in the deep Indian at 14.5 (14.1-15.0) kyr B.P. The termination onset occurs at 17.5 kyr B.P. in the intermediate and deep North Atlantic, deep South Atlantic, and deep Pacific. However, throughout the termination deep North Atlantic benthic δ18O leads the deep Pacific by an average of 1000-year and a maximum of 1700-year. Additionally, the intermediate Pacific termination onset at 16.5 (16.1-16.9) kyr B.P. demonstrates that intermediate-depth benthic δ18O change was not globally synchronous. These regional stacks provide better age models than a global stack across Termination 1 and potentially important constraints on deglacial ocean circulation changes. Key Points Seven regional benthic δ18O stacks with 14C age models Termination 1 onset at 18.5 ka in intermediate South Atlantic and 14.5 ka in deep Indian Deep Pacific lags deep Atlantic by average of 1 kyr during Termination 1
          Cover page: Termination 1 timing in radiocarbon‐dated regional benthic δ18O stacks
          • Article
          • Peer Reviewed
          UC Santa Barbara Previously Published Works (2009)
          Benthic δ 18O is often used as a stratigraphic tool to place marine records on a common age model and as a proxy for the timing of ice volume/sea level change. However, Skinner and Shackleton (2005) found that the timing of benthic δ 18O change at the last termination differed by 3900 years between one Atlantic site and one Pacific site. These results suggest that benthic δ 18O change may not always accurately record the timing of deglaciation. We compare benthic δ 18O records from 20 Atlantic sites and 14 Pacific sites to evaluate systematic differences in the timing of terminations in benthic δ 18O. Analysis of sedimentation rates derived from the alignment of benthic δ 18O suggests a statistically significant Atlantic lead over Pacific benthic δ 18O change during the last six terminations. We estimate an average Pacific benthic δ 18O lag of 1600 years for Terminations 1-5, slightly larger than the delay expected from ocean mixing rates given that most glacial meltwater probably enters the North Atlantic. We additionally find evidence of ∼4000-year Pacific δ 18O lags at approximately 128 ka and 330 ka, suggesting that stratigraphic correlation of δ 18O has the potential to generate age model errors of several thousand years during terminations. A simple model demonstrates that these lags can be generated by diachronous temperature changes and do not require slower circulation rates. Most importantly, diachronous benthic δ 18O responses must be taken into account when comparing Atlantic and Pacific benthic δ 18O records or when using benthic δ 18O records as a proxy for the timing of ice volume change. Copyright 2009 by the American Geophysical Union.
            Cover page: Diachronous benthic δ18O responses during late Pleistocene terminations
            • Article
            • Peer Reviewed
            UC Santa Barbara Previously Published Works (2007)
            A composite section, which reconstructs a continuous stratigraphic record from cores of multiple nearby holes, and its associated composite depth scale are important tools for analyzing sediment recovered from a drilling site. However, the standard technique for creating composite depth scales on drilling cruises does not correct for depth distortion within each core. Additionally, the splicing technique used to create composite sections often results in a 10-15% offset between composite depths and measured drill depths. We present a new automated compositing technique that better aligns stratigraphy across holes, corrects depth offsets, and could be performed aboard ship. By analyzing 618 cores from seven Ocean Drilling Program (ODP) sites, we estimate that ∼80% of the depth offset in traditional composite depth scales results from core extension during drilling and extraction. Average rates of extension are 12.4 ± 1.5% for calcareous and siliceous cores from ODP Leg 138 and 8.1 ± 1.1% for calcareous and clay-rich cores from ODP Leg 154. Also, average extension decreases as a function of depth in the sediment column, suggesting that elastic rebound is not the dominant extension mechanism. Copyright 2007 by the American Geophysical Union.
              Cover page: Automated composite depth scale construction and estimates of sediment core extension
              • Article
              • Peer Reviewed
              UCSB Open Access Publishing Fund Supported Publications (2016)
                Creative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Santa Barbara Previously Published Works (2018)
                Abstract. We present a compilation of 127 time series δ13C records from Cibicides wuellerstorfi spanning the last deglaciation (20–6 ka) which is well-suited for reconstructing large-scale carbon cycle changes, especially for comparison with isotope-enabled carbon cycle models. The age models for the δ13C records are derived from regional planktic radiocarbon compilations (Stern and Lisiecki, 2014). The δ13C records were stacked in nine different regions and then combined using volume-weighted averages to create intermediate, deep, and global δ13C stacks. These benthic δ13C stacks are used to reconstruct changes in the size of the terrestrial biosphere and deep ocean carbon storage. The timing of change in global mean δ13C is interpreted to indicate terrestrial biosphere expansion from 19–6 ka. The δ13C gradient between the intermediate and deep ocean, which we interpret as a proxy for deep ocean carbon storage, matches the pattern of atmospheric CO2 change observed in ice core records. The presence of signals associated with the terrestrial biosphere and atmospheric CO2 indicates that the compiled δ13C records have sufficient spatial coverage and time resolution to accurately reconstruct large-scale carbon cycle changes during the glacial termination.
                  Cover page: Deglacial carbon cycle changes observed in a compilation of 127 benthic δ13C time series (20–6 ka)
                  • Article
                  • Peer Reviewed
                  UC Santa Barbara Previously Published Works (2016)
                    Cover page: Regional and global benthic δ18O stacks for the last glacial cycleCreative Commons 'BY-NC-ND' version 4.0 license
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