Chronology of the pressure-temperature history recorded by a granulite terrain

Cohen, A. S.; O’Nions, R.K.; Siegenthaler, R.; Griffin, William L.

doi:10.1007/bf00375181

Cited by 289 publications

(148 citation statements)

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“…The Nd in the leach solutions was chemically treated and purified following the protocol of Cohen et al (1988). Nd isotope compositions were measured on a Nu Instruments multi collector inductively coupled plasma mass spectrometer at GEOMAR Kiel.…”

Section: Methodsmentioning

confidence: 99%

A major change in North Atlantic deep water circulation 1.6 million years ago

Khélifi

2014

Clim. Past

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Abstract. The global ocean-climate system has been highly sensitive to the formation and advection of deep overflow water from the Nordic Seas as integral part of the Atlantic Meridional Overturning Circulation (AMOC) but its evolution over the Pliocene-Pleistocene global cooling is not fully understood. In particular, changes in the sources and mixing of prevailing deep waters that were involved in driving overturning throughout the Pliocene-Pleistocene climate transitions are not well constrained. Here we investigate the evolution of a substantial deep southward return overflow of the AMOC over the last 4 million years. We present new records of the bottom-water radiogenic neodymium isotope (ε Nd ) variability obtained from three sediment cores (DSDP site 610 and ODP sites 980/981 and 900) at water depths between 2170 and 5050 m in the northeast Atlantic. We find that prior to the onset of major Northern Hemisphere glaciation (NHG) ∼ 3 million years ago (Ma), ε Nd values primarily oscillated between −9 and −11 at all sites, consistent with enhanced vertical mixing and weak stratification of the water masses during the warmer-than-today Pliocene period. From 2.7 Ma to ∼ 2.0 Ma, the ε Nd signatures of the water masses gradually became more distinct, which documents a significant advection of Nordic Seas overflow deep water coincident with the intensification of NHG. Most markedly, however, at ∼ 1.6 Ma the interglacial ε Nd signatures at sites 610 (2420 m water depth (w.d.)) and 980/981 (2170 m w.d.) synchronously and permanently shifted by 2 to 3 ε Nd units to less radiogenic values, respectively. Since then the difference between glacial and interglacial ε Nd values has been similar to the Late Quaternary at each site. A decrease of ∼ 2 ε Nd units at 1.6 Ma was also recorded for the deepest water masses by site 900 (∼ 5 050 m w.d.), which thereafter, however, evolved to more radiogenic values again until the present. This major ε Nd change across the 1.6 Ma transition reflects a significant reorganization of the overturning circulation in the northeast Atlantic paving the way for the more stratified water column with distinct water masses prevailing thereafter.

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Section: Methodsmentioning

confidence: 99%

A major change in North Atlantic deep water circulation 1.6 million years ago

Khélifi

2014

Clim. Past

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“…Strontium and Neodymium were extracted from the FeeMn oxyhydroxide matrix following the standard prodecures of Horwitz et al (1992) for Sr and those of Cohen et al (1988) for Nd. Total analytical blanks were on the order of w32 pg for Sr and below 10 pg for Nd and are negligible.…”

Section: Methodsmentioning

confidence: 99%

Changes in North Atlantic Deep Water strength and bottom water masses during Marine Isotope Stage 3 (45–35kaBP)

Gutjahr

Hoogakker

McCave

2010

Quaternary Science Reviews

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a b s t r a c tThe strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present highresolution Nd isotope compositions of past seawater derived from authigenic FeeMn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481 m water depth). The data cover the period from 45 to 35 ka BP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, w40e39 ka BP). The Nd isotope record suggests that there was no northern-source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35 ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500 m at this location. The 3 Nd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 (3 Nd ¼ À11.3) and most radiogenic immediately preceding IS 9 (3 Nd ¼ À9.8). More radiogenic compositions were also recorded during H4 (À10.2 3 Nd À9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3 ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South.

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“…Numerical analysis of the two age groups suggest that they are statistically different, but caution must be exercised in interpreting the geological significance of this result, especially when the younger group is defined by one analysis only. If the Tc of Sm-Nd system is higher than 800°C, as it was suggested by Cohen et al (1988) and Jagoutz (1988), the group with average SmNd age of 1880 Ma could reflect the peak metamorphic event age since the Tc is higher than the estimated peak conditions (700-750°C) in the studied migmatites. In contrast, the Sm-Nd age of 1843111 Ma obtained for the L2 leucosome vein could be an artefact either due to the small size and scarcity of garnet or due to Sm-Nd fractionation resulting from garnet dissolution during leaching.…”

Section: Sm-nd On Garnet-whole Rockmentioning

confidence: 62%

“…Low Tc (< 700°C) have been proposed e.g. by Mezger et al (1992) and Burton et al (1993) while some other authors have shown evidence of higher temperatures that could be at least 700°C (Hensen & Zhou 1995), about 760°C (Vance & O'Nions 1990) or more than 800°C (Cohen et al 1988, Jagoutz 1988.…”

Section: Review Of Minerals Used and The Problem Of Closure Temperatumentioning

confidence: 92%

Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland

Mouri¹,

Korsman²,

Huhma³

1999

Bull Geol Soc Finland

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which is characterized by Kfs-poor leucosomes and borders the former belt in the north. The present paper deals with selected migmatitic rocks from the latter belt. It is aimed to study the temporal and structural relationships of the different leucosome generations, and to establish the pressure-temperature-time paths of this belt.The Tonalite-Trondhjemite Migmatite Belt consists mainly of migmatitic rocks with various types of synorogenic granitoids and minor mafic and ultramafic rocks. The mesosome of the migmatites consist of garnet-sillimanite-biotite-plagioclase-cordierite-quartz assemblages with rare K-feldspar and late andalusite. The oldest leucosomes are dominated by plagioclase and quartz, and the content of K-feldspar increases in later leucosomes. Microtextural analysis in conjunction with THERMOCALC calculations and geothermometry shows that these rocks were metamorphosed at peak conditions of 700-750°C at 4-5 kbar and aH,0 = 0.4-0.7. The formation of cordierite coronas around garnet and the late crystallization of andalusite suggest that the final stage of the P-T history was characterized by decompression and cooling within the andalusite stability field, estimated at 500-650°C and 3-4 kbar.Detailed isotopic dating of mesosome and leucosomes of the migmatites was undertaken by conventional U-Pb analyses on monazite and zircon, Sm-Nd analyses on garnet, and ion probe dating on zircon. The monazites are nearly concordant with an average age of 1878.5±1.5 Ma, and garnet-whole rock analyses show that the concordant leucosomes and the mesosome are coeval within error margins having ages of 1893±40 and 1871114 Ma, respectively. However, garnet in the discordant vein leucosome provides an age of 1843±11 Ma, which is marginally younger than the age of the adjacent mesosome and the concordant leucosome (1877+18 and 1880±23 Ma, respectively) and the age of monazite. Zircons from the studied migmatites display complex zoning structures using SEM-based CL-imaging. Most grains have distinct cores, clearly remnants of original grains. The cores display various types of zoning but oscillatory zon-32 H. Mouri, K. Korsman and H. Huhma ing dominates. The cores are overgrown by one or two thin outer rims that are of two types: i) unzoned outer rim, considered as overgrowth of new zircon during a metamorphic event, and ii) weakly oscillatory zoned rim, considered as typical of magmatic recrystallization. Ion probe dating of cores yielded slightly discordant 207 Pb/ 206 Pb ages of between 2866-2002 Ma, which are interpreted as protolith age. Rims yielded two major age groups: the unzoned rims gave ages of 1872-1886 Ma, whereas the rims with oscillatory zoning yielded ages of 1951-1959 Ma. The youngest age group is consistent with the conventional Sm-Nd dating on garnet and U-Pb dating on monazite and we suggest that the migmatites were metamorphosed at granulite facies conditions at ca. 1880 Ma. The 1951-1959 Ma age group yielded by magmatic zircon rims remains difficult to interpret, but may reflect a magmatic eve...

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Chronology of the pressure-temperature history recorded by a granulite terrain

Cited by 289 publications

References 21 publications

A major change in North Atlantic deep water circulation 1.6 million years ago

A major change in North Atlantic deep water circulation 1.6 million years ago

Changes in North Atlantic Deep Water strength and bottom water masses during Marine Isotope Stage 3 (45–35kaBP)

Tectono-metamorphic evolution and timing of the melting processes in the Svecofennian Tonalite-Trondhjemite Migmatite Belt: An example from Luopioinen, Tampere area, southern Finland

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