Proxies in Late Cenozoic Paleoceanography

C. Hillaire-Marcel, Anne de Vernal

• Publisher: Elsevier Science
• Year: 2007
• ISBN:
  9780080525044
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

The present volume is the first in a series of two books dedicated to the paleoceanography of the Late Cenozoic ocean. The need for an updated synthesis on paleoceanographic science is urgent, owing to the huge and very diversified progress made in this domain during the last decade. In addition, no comprehensive monography still exists in this domain. This is quite incomprehensible in view of the contribution of paleoceanographic research to our present understanding of the dynamics of the climate-ocean system. The focus on the Late Cenozoic ocean responds to two constraints. Firstly, most quantitative methods, notably those based on micropaleontological approaches, cannot be used back in time beyond a few million years at most. Secondly, the last few million years, with their strong climate oscillations, show specific high frequency changes of the ocean with a relatively reduced influcence of tectonics.

The first volume addresses quantitative methodologies to reconstruct the dynamics of the ocean andthe second, major aspects of the ocean system (thermohaline circulation, carbon cycle, productivity, sea level etc.) and will also present regional synthesis about the paleoceanography of major the oceanic basins. In both cases, the focus is the "open ocean" leaving aside nearshore processes that depend too much onlocal conditions. In this first volume, we have gathered up-to-date methodologies for the measurement and quantitative interpretation of tracers and proxies in deep sea sediments that allow reconstruction of a few key past-properties of the ocean( temperature, salinity, sea-ice cover, seasonal gradients, pH, ventilation, oceanic currents, thermohaline circulation, and paleoproductivity).

Chapters encompass physical methods (conventional grain-size studies, tomodensitometry, magnetic and mineralogical properties), most current biological proxies (planktic and benthic foraminifers, deep sea corals, diatoms, coccoliths, dinocysts and biomarkers) and key geochemical tracers (trace elements, stable isotopes, radiogenic isotopes, and U-series). Contributors to the book and members of the review panel are among the best scientists in their specialty. They represent major European and North American laboratories and thus provide a priori guarantees to the quality and updat of the entire book. Scientists and graduate students in paleoclimatology, paleoceanography, climate modeling, and undergraduate and graduate students in marine geology represent the target audience.

This volume should be of interest for scientists involved in several international programs, such as those linked to the IPCC (IODP – Integrated Ocean Drilling Program; PAGES – Past Global Changes; IMAGES – Marine Global Changes; PMIP: Paleoclimate Intercomparison Project; several IGCP projects etc.), That is, all programs that require access to time series illustrating changes in the climate-ocean system.

• Presents updated techniques and methods in paleoceanography
• Reviews the state-of-the-art interpretation of proxies used for quantitative reconstruction of the climate-ocean system
• Acts as a supplement for undergraduate and graduate courses in paleoceanography and marine geology

Chapter One Deep-Sea Sediment Deposits and Properties Controlled by Currents

I.N. McCave^*,

^* Corresponding author.

E-mail address: [email protected]

Publisher Summary

This chapter focuses on the deep-sea sediment deposits and properties controlled by currents. Initial recognition of the importance of deep currents in redistributing sediments came from the combination of: (1) photographic evidence for bedforms under known currents, (2) photographic evidence for bedforms under known currents, (3) acoustic profiler data from echo-sounder and 3.5 kHz, and (4) seismic reflection profiles documenting large sediment bodies under current systems. Deep-sea sediments normally show few structures other than biological disturbance, therefore, grain size parameters have been used as the best indicator of relative flow speed. The deep circulation does not deliver sediment to the ocean, but reworks it once it has arrived. Sediment delivery is mainly by gravity-driven processes. In the open ocean far from land, pelagic sinking flux is rapid—on a timescale of several tens of days — because of particle aggregation and biological packaging. Close to continental margins, sediment is carried downslope in copious quantities by turbidity currents and debris flows that are responsible for construction of much of the continental rise. Gravity flows do not deliver sediment bearing a pure signature characteristic of conditions in the source area at the time they were triggered. Strong deep-sea currents resuspend this sediment, forming nepheloid layers, and move it to areas of spatial decrease in flow speed, causing deposition. The chapter discusses the biological indicators of flow speed, global ocean flow patterns, sediment transport and deposition by deep-sea currents, and current problems and prospects.

1 Introduction

Sediments carry diverse types of information in their composition and grain size. An enormous number of chemical and isotopic attributes have been used to deduce environments of deposition and sediment production with particular emphasis on climatic variables. In many instances the bulk of the sediment is just seen as a carrier phase for the main information provider, often foraminifera. This attitude has led to several interpretational problems, e.g. in age modelling, which could be avoided by considering the whole sediment and the processes by which it is delivered, sorted and deposited. This topic and the interpretation of the vigour of the depositing flow are treated here.

1.1 Current Indicators in Deep-Sea Sediments

Initial recognition of the importance of deep currents in redistributing sediments came from the combination of:

1) Photographic evidence for bedforms under known currents (Heezen & Hollister, 1964),

2) Sedimentary structures in cores under strong currents (Hollister & Heezen, 1972),

3) Acoustic profiler data from echo-sounder and 3.5 kHz (Schneider et al., 1967), and

4) Seismic reflection profiles documenting large sediment bodies under current systems (Johnson & Schneider, 1969; Jones, Ewing, Ewing, & Eittreim, 1970).

1.1.1 Photographs

Systematic regional examination of photographs shows spatial variability of flow (Tucholke, Hollister, Biscaye, & Gardner, 1985) (Figure 1) and has yielded an ordinal scale of current intensity shown by photographed bedforms (McCave & Tucholke, 1986). Although these show the zonation of current speed very well by progression from subtle smoothing through longitudinal ripples to barchan ripples and crag-and-tail structure, this is virtually useless below the surface. The pervasive structure of muddy contourite sediments is bioturbation (Wetzel, 1991; Baldwin & McCave, 1999; Löwemark, Schönfeld, Werner, & Schafer, 2004 show X-radiographs). This destroys virtually all traces of original depositional structure.

Figure 1 Current zonation of Nova Scotian Rise deduced from bedforms seen in bottom photographs. Based on data in Tucholke et al. (1985). Key to photographic indicators of increasing flow speed: W/T=weak/tranquil, INT=intermediate, LR=longitudinal ripples, Cloudy=photos showing turbid water due to resuspension. The HEBBLE instrument deployments and cores were around 40°27′N, 62°20′ W.

1.1.2 Sedimentary structures

A few contourites show clear sedimentary structure (Figure 2), but these are relatively sandy (e.g. shown is 60–80% >63 μm). The sand (mainly foraminifera) content of many contourites is in the region of 10–20%, decreasing with increasing accumulation rate. In the case of a S.W. Indian Ocean core WIND 27K the sand percentage is 25–40% with an accumulation rate of ∼3 cm ka⁻¹ for the period 0–60 ka. Over a 20 cm thick section shown in Figure 2, the sand percentage goes up to 80% and the structure is ripple cross-lamination. Our best estimate is that this section represents from 125 to 60 ka, a mean sedimentation rate of 0.3 cm ka⁻¹, considerably less and certainly not continuous. The current is inferred to have strengthened sufficiently to rework the section from stages 4 down to 5e, possibly in a couple of high-speed pulses lasting a few thousand years at the 5e–5d and 5a–4 transitions (McCave, Kiefer, Thornalley, & Elderfield, 2005). This demonstrates the fact that the stratigraphic resolution of sandy contourites is likely to be shot through with hiatuses. High (foram) sand contents are generally due to either dissolution of fine carbonate or winnowing removal of fine sediment.

Figure 2 X-radiograph of core WIND 27K, 165-198 cm, from the Amirante Passage, equatorial western Indian Ocean, showing cross-laminated contourite muddy foraminiferal sand. Note that the base is sharp but deformed during coring and that the top is gradational in texture. Fine-scale lamination, cross lamination and erosion surfaces are evident. This 20 cm represents around 40,000 yr, demonstrating the stratigraphic penalty of too-high current speed.

1.1.3 Acoustic profiles

Mapping of echo character from 10 kHz sounders also showed current zonation on the margin (Hollister & Heezen, 1972), but this was superseded by the now ubiquitous 3.5 kHz whose interpretation was systematised by workers at Lamont in the 1970s (Damuth, 1980). The combination of high frequency echo sounder (10–20 kHz) and 3.5 kHz profiler can be revealing (Figure 3) (Winn, Kogler, & Werner, 1980), because the strength of the acoustic return is related to the surface sediment density (porosity) and grain size, which is a function of net accumulation rate, involving both deposition and winnowing. As can be seen from Figure 3, there is a spatially coherent response of sedimentation rate and reflectivity (mainly porosity) to current flow.

Figure 3 (Upper) 3.5 kHz WNW-ESE profile across northern Gardar Drift at 60° N, 23° W showing reduced sedimentation rate on the eastern slope of the drift by closer spaced reflectors. (Lower) 10 kHz echo sounder record along the same track showing higher amplitude reflection (redder colour) of the ‘harder’ sea bed more strongly affected by currents, corresponding to coarser...

Table of contents

• Cover image
• Title page
• Table of Contents
• Dedication
• Contributors
• Scientific Committee
• Introduction Methods in Late Cenozoic Paleoceanography: Introduction
• Chapter One Deep-Sea Sediment Deposits and Properties Controlled by Currents
• Chapter Two Continuous Physical Properties of Cored Marine Sediments
• Chapter Three Magnetic Stratigraphy in Paleoceanography: Reversals, Excursions, Paleointensity, and Secular Variation
• Chapter Four Clay Minerals, Deep Circulation and Climate
• Chapter Five Radiocarbon Dating of Deep-Sea Sediments
• Chapter Six Planktonic Foraminifera as Tracers of Past Oceanic Environments
• Chapter Seven Paleoceanographical Proxies Based on Deep-Sea Benthic Foraminiferal Assemblage Characteristics
• Chapter Eight Diatoms: From Micropaleontology to Isotope Geochemistry
• Chapter Nine Organic-Walled Dinoflagellate Cysts: Tracers of Sea-Surface Conditions
• Chapter Ten Coccolithophores: From Extant Populations to Fossil Assemblages
• Chapter Eleven Biomarkers as Paleoceanographic Proxies
• Chapter Twelve Deep-Sea Corals: New Insights to Paleoceanography
• Chapter Thirteen Transfer Functions: Methods for Quantitative Paleoceanography Based on Microfossils
• Chapter Fourteen Elemental Proxies for Palaeoclimatic and Palaeoceanographic Variability in Marine Sediments: Interpretation and Application
• Chapter Fifteen Isotopic Tracers of Water Masses and Deep Currents
• Chapter Sixteen Paleoflux and Paleocirculation from Sediment 230T and 231P/230T
• Chapter Seventeen Boron Isotopes in Marine Carbonate Sediments and the pH of the Ocean
• Chapter Eighteen The Use of Oxygen and Carbon Isotopes of Foraminifera in Paleoceanography
• Chapter Nineteen Elemental Proxies for Reconstructing Cenozoic Seawater Paleotemperatures from Calcareous Fossils
• Conclusion Reconstructing and Modeling Past Oceans
• Index of Taxa
• Subject Index