Paleoclimate time series: New alignment and compositing techniques, a 5.3-Myr benthic d18O stack, and analysis of Pliocene-Plesitocene climate transitions
Paleoclimate research has recently produced an explosion of new time series, but fully utilizing the acquired data requires coordinated analysis of globally distributed records. To this end, I developed an automated graphic correlation technique, which uses dynamic programming to find the globally optimal alignment of two paleoclimate time series. Geologic realism is instilled in the solution through penalty functions based on the records' relative accumulation rates. I also adapted the algorithm to automate the generation composite depth sections for marine sediments cores and to characterize patterns of core deformation. The new compositing algorithm addresses two shortcomings of the standard splicing technique (Hagelberg et al., 1992): (1) a failure to correct for distortion within cores, so that one sedimentary feature may have a slightly different composite depth in each hole, and (2) the tendency to produce composite depths which are ~10% greater than recorded drill depths. Analysis of 618 cores from ODP Legs 138 and 154 shows that average extension due to drilling and recovery is 11.5% in carbonate and siliceous sediment and 6.3% in terrigenous-dominated sediment. For both legs, average extension is greatest 1-2 m below the core top, but the exact pattern of deformation varies as a function of drilling depth and apparatus.
A 5.3-Myr stack of 57 aligned benthic d18O records (the "LR04" stack), which describes changes in global ice volume and deep ocean temperature and an improved d18O age model provide the paleoclimate community with important stratigraphic tools to aid in the comparison of widely distributed marine climate records. The LR04 stack is the first composed of more than three benthic records to extend beyond 850 ka, and its improved signal quality is used to identify 24 new marine isotope stages in the early Pliocene. The LR04 stack also contains significantly more variance in the late Pleistocene than previously published stacks due to higher-resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. An improved Pliocene-Pleistocene age model is created by tuning the stack to a simple ice model based on June 21 insolation at 65°N with constraints provided by stacked sedimentation rates. The relative phases of obliquity and precession response in the LR04 stack suggest that northern insolation has been the major driver of d18O change since at least 4.1 Ma and that some change in the phase of precession or obliquity response occurred at approximately 1.4 Ma.
Finally, I studied the evolution of d18O response over the last 5.3 Myr in terms of mean, variance, spectral density, and the shape of glacial cycles. The variance of d18O exhibits an exponential increase with time and a strong response to long-term modulations in orbital forcing. The detrended variance of d18O in the 41-kyr band has a strong correlation with obliquity modulation until 1.4 Ma. The precession components of d18O are strongly correlated with precession modulation throughout the Pliocene and Pleistocene. However, variance in the 100-kyr band of d18O is anticorrelated with modulation of the 100-kyr eccentricity cycle and is not unique to the late Pleistocene. The dominance of the 100-kyr cycle in the late Pleistocene can be modeled as the extension of trends which began before the onset of northern hemisphere glaciation (NHG), namely a steady increase in 100-kyr variance and its sensitivity to 100-kyr power in eccentricity. Neither the onset of NHG nor the mid-Pleistocene "revolution" is associated with unique or abrupt transitions in the spectral response of d18O relative to insolative forcing. However, a climate transition at 1.4 Ma results in a sudden change in the shape of glacial cycles, a trend toward warmer interglacials, reduced sensitivity to obliquity modulations, and a change in the relative lags of precession and obliquity responses in benthic d18O.