Paleoceanography of the southern Coral Sea across the Mid-Pleistocene Transition
View/ Open
Data.zip (107.2Kb)
Published_papers.zip (4.958Mb)
Thesis word.zip (16.84Mb)
Date
2011Author
Russon, Thomas Ford
Metadata
Abstract
A comprehensive theory explaining the relationship between periodic
variations in the Earths orbital parameters and the response of the climate system
remains elusive. One of the key challenges is that of the Mid-Pleistocene Transition
(MPT), during which the dominant mode of glacial/interglacial climatic variability
shifted without any corresponding change in the mode of orbital forcing. Subtropical
climate on orbital time-scales is sensitive to variability in both the low-latitude
ocean/atmosphere circulation regime and the global carbon-cycle (through its effect
on atmospheric greenhouse gas levels), both of which may have played a role in the
shift in mode of global climate response to orbital forcing during the MPT. This
thesis presents a series of multi-proxy (foraminiferal stable isotope and trace-metal)
paleoceanographic reconstructions from the subtropical southwest Pacific, as seen in
marine sediment core MD06-3018, from 2470m water depth and 23ºS in the New
Caledonia Trough, southern Coral Sea. The core age-model, based upon magnetic
stratigraphy and orbital tuning, yields a mean sedimentation rate at the site of
20mm/ka and a core-bottom age of 1600ka.
The MD06-3018 reconstruction of New Caledonia Trough deep water
chemistry, based on benthic 13C measurements, shows that the spatial chemistry
gradient within the Southern Ocean between deep waters entering the Tasman Sea
and the open Pacific was greater during glacial (relative to interglacial) stages over at
least the past 1100ka. This gradient was, however, generally reduced on the >100kyr
time-scale across the MPT, consistent with it being a period of reduced deep water
ventilation in both hemispheres.
The MD06-3018 Mg/Ca-derived reconstruction of subtropical southwest
Pacific Sea Surface Temperature (SST) shows glacial/interglacial variability of 2-3ºC
but no significant trends on the >100kyr time-scale over the duration of the record.
An estimate of the uncertainty associated with the SST reconstruction demonstrates
that no significant changes in reconstructed southern Coral Sea mean-annual SST can
be identified between interglacial stages across the MPT. It is, therefore, unlikely that regional climatic change constituted the main cause for the observed middle
Pleistocene expansion of coral reef systems. The >100kyr time-scale stability of
southern Coral Sea SST means that the position of the southern boundary of the
Pacific warm pool has also been stable over at least the past 1500ka. Comparison
with other low-latitude Pacific reconstructions shows that the early Pleistocene warm
pool was consequently more hemispherically asymmetric than its present
configuration, with the latter being established by ~1000ka and implying significant
changes in meridional atmospheric heat and moisture fluxes prior to the MPT.
On orbital time-scales, the SST reconstruction shows a clear shift from
dominant 40kyr to 100kyr modes of variability over the MPT, although significant
40kyr structure is also retained into the middle/late Pleistocene. In contrast,
reconstructed hydrological cycle variability (based on coupled 18O-Mg/Ca
measurements) shows only limited coherence with the obliquity cycle and a stronger
relationship with the precession cycle. The decoupling of the reconstructed
subtropical SST and hydrological cycle responses places constraints on the extent of
orbitally paced fluctuations in the low-latitude ocean/atmosphere system. Instead,
comparison of the MD06-3018 SST reconstruction with others from across the lowlatitude
Pacific supports a dominant role for greenhouse gas forcing in low-latitude
western Pacific glacial/interglacial SST variability across the Pleistocene.
The subtropical multi-proxy climate reconstructions presented here show that
the timing and sense of long-term (>100kyr time-scale) changes in the low-latitude
ocean/atmosphere circulation regime are consistent with that system having been
important in the expansion of northern hemisphere ice-volume during the early part
of the MPT. However, the subtropical reconstructions also suggest that neither the
low-latitude ocean/atmosphere circulation system nor the global carbon-cycle
underwent a fundamental change in mode of response to orbital forcing during the
transition. Instead, the origin of the 100kyr glacial/interglacial mode was most likely
related to thresholds in the dynamics of the expanding northern hemisphere icesheets,
leading in turn to the existence of significant inter-hemispheric asymmetry in
the orbital time-scale climate response over the middle/late Pleistocene.
Summary for Non-Specialists.
Over the past five million years of its history, the Earths climate has
undergone a series of regular, or nearly regular, fluctuations between warmer and
colder states. These fluctuations take tens to hundreds of thousands of years to occur
and are known as the ‘glacial/interglacial cycles’ on account of the associated
changes in ice-sheet extent in the high-latitudes. The origin of these cycles is widely
held to be the regular variations in form of the Earths orbit around the sun. In spite of
decades of research, however, no complete ‘orbital theory of climate’ exists, mainly
because the patterns of past climate variability, as reconstructed using ‘proxies’ for
variables such as surface temperature, is much more complex than that of the orbital
variations themselves. It follows that processes within the Earth system, especially
those associated with large ice-sheets, the carbon-cycle and the ocean circulation
system, act to substantially modify the climate response to the orbital variations.
Over the past ten years, new observations from both ice-cores and low-latitude
marine sediment cores have suggested that the dominant system(s) involved in
setting the Earths response to the orbital variations may potentially be the carboncycle
and/or the low-latitude ocean/atmosphere circulation regime rather than highlatitude
ice-sheet dynamics, as was generally supposed previously. If this new view
is correct, it has profound implications for the general sensitivity of the climate to the
carbon-cycle on a range of time-scales - making its evaluation a scientific objective
of considerable current importance.
This thesis presents a series of reconstructions of aspects of climate and
carbon-cycle variability for the subtropical southwest Pacific, as based on proxy
measurements in a marine sediment core than spans the past 1,600,000 years at
around 5000 year resolution. The key focus is on an interval called the ‘Mid-
Pleistocene Transition’, during which time the mode of glacial/interglacial variability
changed, indicating a fundamental change in one or more aspects of the response to
the orbital forcing. The study site is well placed to investigate variability in both the
carbon-cycle and low-latitude ocean circulation over the climatic transition as it lies
between the Southern Ocean, a key source of carbon-cycle variability and the equatorial Pacific, where the modern El-Niño system arises. By characterizing
variability in these systems, the potential role played by both systems in causing the
change in mode of glacial/interglacial variability can be evaluated.
The key findings of the thesis are that; firstly, changes in the long-term state
of the low-latitude ocean circulation system may well have been important for the
expansion of northern hemisphere ice-sheets during the early part of the Mid-
Pleistocene Transition. Secondly, it provides further support for a close connection
between variability in the carbon-cycle and low-latitude climate on orbital timescales
but suggests that there is no clear evidence for either system undergoing a
fundamental change in sensitivity to the orbital forcing during the transition. This
implies that whilst paleoclimate records can potentially provide useful constraints on
climate sensitivity to greenhouse gas forcing, it was the changing dynamics of the
newly expanded northern hemisphere ice-sheets that led to the change in dominant
glacial/interglacial periodicity during the Mid-Pleistocene Transition.