Sediment Composition Changes in the Prydz Bay Continental Margin, Antarctica – Implications for Paleoclimate

Kari Strand, Juho Junttila, Jarkko Lamminen, Jari Näsi, Tuija Siira, Karla Tiensuu & Henna Valppu

IODP research team at the University of Oulu, Finland

17th ISC 2006 Session TS11-5 “Sedimentary record on paleoceanography and paleoclimatologyAugust 27 – September 1, 2006, Fukuoka, Japan

Antarctic ice sheet evolution is important for global oceanic circulation and climate

A study of sediment sequence development on the continental margin off Prydz Bay, Antarctica obtained by ocean research drilling (ODP)

Aims are in getting further understanding for onset of glaciation in East Antarctica and look for signals of possible dynamic phases in relatively stable East Antarctic Ice Sheet (EAIS)

Study of sediment composition changes and glacial sedimentary regimes

Microtextural analysis of quartz sand grains for the earliest glacial influence

Heavy mineral analysis for provenance changes in preglacialand glacial sediments

Clay mineral analysis for possible dynamic behaviour of ice sheet

Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K. (2001). Science 292, 686-693.

Global Climate Evolution

A

B

ASite 1166: Late Eoceneobjectives

BSite 1165: Mid Miocene -Mid Pliocene objectives

ODP Leg 188 (Prydz Bay) Sites

Continental Margin off Prydz Bay

Site 1166 Lithostratigraphy

Site 1166 Sediment Compositions

Angular outline; straight grooves Edge abrasion Straight and arguate steps

High relief (clay-rich interval)

Linear fractures and fractured plates

Rounded outlines Rounded outline with later imbricated blocks

Chemically weathered surfaces

Diagnostic microtextures in quartz sand grains

Samples were taken in fraction between 0.250 and 0.6 mm

Frequency (%) from selected samples of Site 1166

From each sample20-30 grains wereanalyzed andcounted by usingSEM-EDS andFESEM at theInstitute of ElectronOptics at theUniversity of Oulu

Angular outlines,edge abrasion,are the mostfrequent inglacial deposits.

Frequency oflarge breakageblocks rangesin wide spectrum(10-40%) indiamictons asthe fluvial sandsshow constantvalues around20%

Microtextural analysis of quartz grains may well be used in verifying for critical periods of ice-sheet evolution such as the transitions from East Antarctic preglacial to glacial conditions

Earliest recovered evidence of nearby mountain glaciation is seen in middle to late Eocene–aged grain textures in fluvial sands

The inland ice sheet may reached sea level as early as ~37 Ma

Summary

from Strand, Passchier & Näsi, 2003, Paleogeogr. Palaeoclimat. Palaeoecol. 198, 101-111

Site 1166 is ideal, since it records a wide timespan and extends back into the non-glaciated time

Provenance is studied by heavy mineral geochemistry

125-250 micron fraction was separated using heavy liquids and mounted in epoxy

Approximately 40 randomly selected grains per sample were analysed by electron microprobeat the Institute of Electron Optics at the University of Oulu

garnet, ilmeniteand amphibole arethe most commonphases

72% granulitic

55% granulitic

Hiatus

(Facies fields after Miyashiro, 1953)

• Garnet content increases steadily upsection• Relative amount of granulite facies garnets increase simultaneously with the

garnet content Provenance is more focused on granulitic rocks.

• Very low Al-composition (mostly <0.08%) is strikingly similar with those analysed from typical granulite belts

• High Ti-content in ilmenite reflect a metamorphic source. Average Ti-content 51% in Prydz Bay ilmenites.

Most likely provenance of ilmenites is granulite source rocks

Garnet

Ilmenite

• Calcic amphibole composition is Mg-rich• Appearance of Fe-Mg amphiboles above the hiatus indicates a possible new

provenance Amphibole suggests a mafic source, probably from amphibolites

Amphibole

The basement geology of the northern Prince Charles Mountainsis dominated by felsic gneisses and metasediments,as well as minor mafic granulites. Metamorphic grade isupper amphibolite – granulite.

The observed metamorphic grade decreases from lower granulite faciesin the north to upper amphibolite facies in the south, continuing to the loweramphibolite to greenschist facies metamorphic grade in the southern Prince Charles Mountains which is dominantly composed of Proterozoicmetasediments and Archean gneisses.

The Lambert Graben is filled by Phanerozoic sediments up to several kilometers.

(Map after Mikhalsky et al. 1999)

In our model we assume that the pre-glacial fluvial systems occupied the Lambert graben and other smaller riftogenic structures.

Early provenance was rather mixed:

-Northern Prince Charles mountains

- Southern Prince Charles mountains

- Phanerozoic sediments in the Lambert Graben

Eroding metasediments in the south and in the graben provided abundant zircons

High grade

Low grade

Ice frozen to the base at southern Prince Charles mountains. No detritus from this area anymore.

Granulite terrains closer to the ice-margin supplied most of the sediment. resulted in high garnet content

Clay mineral distribution from the last 5 Ma (Site 1165)

Decreasing smectite content in consistent with general cooling phases.

Kaolinite is derived from the older weathered soils and indicate glacial erosion.

Smectite increase in the Mid-Pliocene time may indicate polythermal phase in ice sheet history

from Junttila, Ruikka & Strand 2005, Global and Planetary Change 45, 151-163

Smectite concentrations significantly fluctuate throughout the section but illite concentrations decreases e.g. in the time interval ~11.5 – ~10.4 Ma.

Summary of clay mineralogy study

In a climate history perspective, the Neogene period represents a time of progressive cooling of the Earth, although there were global warm periods during that, possible also introducing weak but detectable signals in glacial sedimentary regimes proximal to EAIS off Prydz Bay.

The variabilities in sediment composition e.g. in clay mineral assemblages in the Prydz Bay continental rise seem to record dynamic behaviour of EAIS and reduced shelf ice in the Middle Miocene and Middle Pliocene time periods.

Clay mineral assemblages integrated with other paleoenvironmental data can improve our knowledge of the past Antarctic glacial evolution and climate.

The Antarctic continent and its margins in future focus

Recently the Integrated Ocean Drilling Program (IODP) is providing us a good opportunity for resolving the scientific objectives of earth sciences also in high latitudes

New values can be obtained and discussion stimulated by carrying out comparative studies of Circum Antarctic, e.g. the Weddel Sea and Wilkes Land margins