A reference time scale for the study of Pleistocene orbital and millennial-scale climate variability: IODP Site U1385 ("Shackleton site")

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

Future advances in our understanding of the Earth's climate system will rely on our ability to link high-resolution sedimentary archives from the oceans, ice-cores and terrestrial sequences, and to interpret these records in the context of novel Earth system modeling approaches. Few places exist in the world where sufficiently detailed and unambiguous marine-ice-terrestrial linkages are possible. The western Iberian Margin represents such a location, and provides a rare opportunity for recovering key sequences needed to for marine-ice-terrestrial correlation. During Expedition 339, we drilled five holes at Site U1385 ("Shackleton site") to a maximum depth of 166.5 mcd, and recovered a continuous record of hemipelagic sedimentation from the Holocene to 1.42 Ma (Marine Isotope Stage 46) with average sedimentation rates of ~10 cm kyr-1. The high-resolution core logging, colour, and XRF data already obtained demonstrate the great potential of this site for studying the history of Pleistocene climate change on orbital and suborbital time scales. Before these records can be fully exploited, however, a well-founded chronology must be developed for Site U1385. Here we propose to develop such a detailed reference time scale to which all future studies of Site U1385 will refer. A robust time scale is particularly important as the Shackleton site is likely to become a "marine reference section" for global correlation and against which other climate records will inevitably be compared.
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Once the age model has been established, it can be transferred to the various time series we have already collected (e.g., magnetic susceptibility, density, NGR, scanning XRF, and colour) and those that will be produced by this project (planktonic and benthonic d18O and d13C, benthic Mg/Ca) and our project partners (alkenone and foraminiferal sea surface temperature estimates). Time series analysis will be used to address several fundamental questions regarding the coevolution of Pleistocene orbital and suborbital climate variability under changing orbital and glacial boundary conditions:

Orbital: What are the phasings of proxy climate variables at Milankovitch frequencies and what do they tell us about the response of different components of the climate system to insolation forcing? How do the observed phasings compare with the system model proposed by Imbrie et al. (1992) detailing the geographic progression of local responses to insolation forcing? Has the phasing changed through the Pleistocene when the average climate state evolved toward generally colder conditions with larger ice sheets, and the spectral character of climate variability shifted from dominantly 41 to 100 kyrs?

Suborbital: How does the frequency and pacing of millennial-scale variability vary among glacial periods of the last 1.4 Ma? Is the amplitude and/or frequency of millennial variations related to an ice volume threshold? Does Earth's orbital geometry influence the nature (magnitude and/or frequency) of suborbital climate variability (e.g., through frequency modulation)? How did millennial-scale climate variability change across the Middle Pleistocene transition? Do millennial-scale climatic oscillations play a role in initiating glacial terminations or are they principally orbitally driven?

The results of this project will benefit the many scientists of IODP Expedition 339 working on Site U1385 by establishing a chronostratigraphic foundation for this important marine reference section.

Planned Impact

The primary non-academic beneficiaries of this work will include educationalists and policy-makers. Benefits will accrue to these stakeholders through emerging insights into the operation of the climate system. These insights may ultimately inform on anthropogenic climate change and may help in developing appropriate responses to the challenges that climate change will raise in the medium term.

Benefits will also accrue to the wider public through the provision of case studies of past environmental change. Such case studies are particularly useful for engaging with members of the public, and thus fostering public understanding of the science of climate change. Science-society interfaces that are available to us include the Department of Earth Sciences undergraduate-led 'Time Truck' (http://www.timetruck.co.uk/), the Sedgwick Museum at the University of Cambridge (which attracts over 84,000 visitors per year), and the Cambridge Science Festival (http://comms.group.cam.ac.uk/sciencefestival/). These science-society interfaces will permit us to engage with a wide range of non-academic beneficiaries, from a range of social backgrounds and age groups. Only by capturing the public imagination through avenues such as these, can real progress be made in fostering greater public understanding of earth system science. Ultimately this can benefit health, well-being, and economic innovation in the UK through long-term influences on the educational and economic choices of our future leaders of industry and future entrepreneurs.

Publications

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Hernández-Molina F (2016) Virtual special issue on IODP Expedition 339: The Mediterranean outflow in Marine Geology

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Hodell D (2016) Virtual special issue on IODP Expedition 339: The Mediterranean outflow in Global and Planetary Change

 
Description We produced a composite depth scale and chronology for Site U1385 for the so-called "Shackleton site" on the SW Iberian Margin. Using Ca/Ti measured by core scanning XRF at 1-cm resolution in all holes, a composite section was constructed to 166.5 meters composite depth (mcd) that corrects for stretching and squeezing in each core. Oxygen isotopes of benthic foraminifera were correlated to a stacked d18O reference signal (LR04) to produce an oxygen isotope stratigraphy and age model.

Variations in sediment color contain very strong precession signals at Site U1385, whose amplitude modulation by eccentricity provides a powerful tool for developing an orbitally-tuned age model. We tuned the U1385 record by correlating peaks in L* to the local summer insolation maxima at 37oN. The benthic d18O record of Site U1385, when placed on the tuned age model, agrees well with LR04 and EDC3 within their chronologic uncertainties. The tuned benthic d18O signal of Site U1385 leads slightly the d18O of Pacific Site 677 on some terminations, reflecting the diachronous d18O response among ocean basins on the order of several thousand years.

The age model is transferred to down-core data to produce a continuous time series of XRF elemental ratios. Variations in log(Ca/Ti) reflect changes in weight percent carbonate and are controlled by the relative proportion of biogenic and detrital sediment. Ca/Ti increases during interglacial and interstadial climate states and decreases during glacial and stadial periods. Much of the variance in the log(Ca/Ti) is explained by a linear combination of orbital frequencies (precession, tilt and eccentricity). The residual signal reflects suborbital climate variability. The strong correlation between Ca/Ti and Greenland temperature over the last glacial cycle at Site U1385 suggests the residual Ca/Ti signal can be used as a proxy of millennial-scale climate variability for the past 1.5 Ma.
Exploitation Route The composite depth and age models developed as part of this project will serve as the stratigraphic and chronologic backbone for paleoceanographic studies by other investigators.
Sectors Environment

 
Description The composite depth and age models developed as part of this project will serve as the stratigraphic and chronologic backbone for paleoceanographic studies by other investigators. It has been made available to Expedition 339 scientists prior to publication.
First Year Of Impact 2014
Sector Environment
 
Description Sedgwick Museum Exhibit -- Geology from the oceans: unlocking the history of climate from the bottom of the sea 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact We developed an exhibit titled: 'Geology from the oceans: unlocking the history of climate from the bottom of the sea' in collaboration with the staff of the Sedgwick Museum at the Earth Sciences Department, University of Cambridge (http://www.sedgwickmuseum.org/). The Sedgwick Museum attracts over 100,000 visitors per year, is a popular destination for school and community groups, and provides an excellent venue for showcasing research. Exhibit is ongoing and total numbers and impacts won't be known until it has closed."How can we understand the history of the Earth's oceans by studying microfossils in columns of mud from the bottom of the sea? This exhibition, which focuses on researchers from the Department of Earth Sciences, University of Cambridge, looks at the way in which sediments from the sea floor have been used over the last fifty years to discover more about the history of the planet. The exhibition explores the Ice Ages that have dominated climate change over the last one million years and looks at how drilling engineering, mass spectrometry, and the Earth's orbit are all ingredients of this remarkable story."
Year(s) Of Engagement Activity 2017,2018
URL http://www.sedgwickmuseum.org