Glacial North Atlantic: Sea surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX-28) modern analog technique with 947 modern analog samples and 119 well-dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice-free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45° and 60°N, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12°C. In the subtropical ocean gyres the new reconstruction supports the glacial-to-interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2° and 6°N suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6°-8°C during Northern Hemisphere summer. Differences between these SIMMAX-based and published U37[k]- and Mg/Ca-based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

(Table 1) Occurrence of selected planktonic foraminifers, other microfossils, and lithologic components at DSDP Hole 96-619

Site 619, located in the Pigmy Basin off the coast of Louisiana, penetrated the late Quaternary Ericson Zones X, Y, and Z. The penetrated section can be divided into four intervals. The lower interval (below 157 m sub-bottom) comprises 51 m of displaced sediments which probably originated from the Louisiana continental shelf. The upper three intervals (above 157 m) are dominated by pelagic/hemipelagic sedimentation associated with a closed basin. These are divided on the basis of planktonic foraminifers into Zones X, Y, and Z. These warm-cool water intervals are identified mainly by using the Globorotalia menardii complex (warm) and G. inflata (cool). The intervals correlate with published curves taken from piston core samples in the western Gulf of Mexico.

Distribution of planktonic foraminifera in DSDP Hole 31-296 (Table III)

Late Neogene planktonic foraminiferal biostratigraphy of DSDP Site 296, Leg 31, reveals this site as an ideal reference section for correlation of Blow's low-latitude zonation with the mid-latitude zonation for temperate faunal assemblages developed in this paper and earlier for DSDP Site 310, Leg 31 (Keller). Abundance of temperate species of Globorotalia (G. inflata, G. puncticulata, G. crassaformis, G. conomiozea) permit correlation with the zonal subdivision developed at Site 310 based on these species. Evolutionary changes within the Globorotalia inflata group also appear to be consistent biostratigraphie markers in mid latitudes; a primitive variety of this species first appears at about 3.3-3.1 Ma, G. inflata praeinflata appears at about 2.6 Ma, and the modern form appears at about 2.2-2.1 Ma. Quantitative analyses of planktonic foraminifera at DSDP Site 296 reveal an inversely reciprocal frequency oscillation between species of Globorotalia and the Globigerina-Globigerinita group. Cool climatic periods are characterized by high frequencies in the Globigerina-Globigerinita group and low frequencies in the Globorotalia group, whereas warm intervals are marked by high frequencies in the Globorotalia group and low frequencies in the Globigerina-Globigerinita group. Five cool paleoclimatic events can be recognized between early Pliocene and late Pleistocene: 4.4 Ma, 3.2-3.1 Ma, 2.4-2.2 Ma, 1.2 Ma, and 0.7 Ma. These paleoclimatic/paleoceanographic events have also been recognized in planktonic foraminifera of the Central and Northeast Pacific DSDP Sites 310 and 173 and also correlate to cold events recognized in oxygen isotope measurements of DSDP Site 310 and in equatorial Pacific cores.

(Table 1) Relative abundance of planktonic foraminifers in ODP Hole 164-997A sediments

Hole 997A was drilled during Leg 164 of the Ocean Drilling Program at a depth of 2770 m on the topographic crest of the Blake Ridge in the western Atlantic Ocean. We report here an analysis of the faunal assemblages of planktonic foraminifers in a total of 91 samples (0.39-91.89 mbsf interval) spanning the last 2.15 m.y., latest Pliocene to Holocene. The abundant species, Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, Globorotalia inflata, and Globigerinita glutinata together exceed over ~70% of the total fauna. Each species exhibits fluctuations with amplitudes of 10%-20% or more. Despite their generally low abundance, the distinct presence/absence behavior of the Globorotalia menardii group is almost synchronous with glacial-interglacial climate cycles during the upper part of Brunhes Chron. The quantitative study and factor analysis of planktonic foraminiferal assemblages shows that the planktonic foraminiferal fauna in Hole 997A consists of four groups: warm water, subtropical gyre (mixed-layer species), gyre margin (thermocline/upwelling species), and subpolar assemblages. The subtropical gyre assemblage dominates throughout the studied section, whereas the abundance of gyre margin taxa strongly control the overall variability in faunal abundance at Site 997. In sediments older than the Olduvai Subchron, the planktonic foraminiferal faunas are characterized by fluctuations in both the subtropical gyre and gyre margin assemblages, similar to those in the Brunhes Chron. The upwelling/gyre margin fauna increased in abundance just before the Jaramillo Subchron and was dominant between 0.7 and 1.07 Ma. The transition from this gyre margin-dominated assemblage to an increase in abundance of the subtropical gyre and gyre margin species occurred around 0.7 Ma, near the Brunhes/Matuyama boundary. The presence of low-oxygen-tolerant benthic foraminifers, pyrite tubes, and abundant diatoms below the Brunhes/Matuyama boundary suggests decreased oxygenation of intermediate waters and more upwelling over the Blake-Bahama Outer Ridge, perhaps because of weaker Upper North Atlantic Deep Water ventilation. The changes in the relative composition of foraminifer assemblages took place at least twice, around 700 and 1000 ka, close to the ~930-ka switch from obliquity-forced climate variation to the 100-k.y. eccentricity cycle. The climate shift at 700 ka suggests a transition from relatively warmer conditions in the early Pleistocene to warm-cool oscillations in the Brunhes Chron.

Relative contents of foraminifera species in bottom sediments of northern and southern seas

Correlation of paleoceanographic events in several key regions of the World Ocean: North Atlantic, Antarctic, West Arctic Seas, North Pacific and tropical Indo-Pacific has been carried out for the last 135 ka based on micropaleontological, stable isotope, geochronological (AMS-14C) and other data. It has been shown that the global thermohaline circulation controls remote climatic teleconnections on millennial-scale and partly on centennial-scale, while short-term climate changes are mainly transferred by the atmosphere. The basic information is given about the recent thermohaline circulation and stages of its development during Neogene.

(Table 1) Relarive abundance of foraminiferal species in the surface layer of bottom sediments from the Atlantic Ocean

Quantitative ratios of tests of planktonic foraminiferal species in thanatocoenoses within the surface layer of bottom sediments from Iceland to the Equator are described. Subarctic, boreal, subtropical and tropical types of thanatocoenoses and their subtypes are distinguished. Each subtype corresponds to a 2-3°C interval of mean annual temperature of the upper layer of ocean water. Comparison of fossil thanatocoenoses from Quaternary sediment cores with recent thanatocoenoses offers new potentials for paleotemperature analysis.

Planktonic foraminiferal faunas from surface-sediment samples from the South China Sea and the western Pacific

We present 30 new planktonic foraminiferal census data of surface sediment samples from the South China Sea, recovered between 630 and 2883 m water depth. These new data, together with the 131 earlier published data sets from the western Pacific, are used for calibrating the SIMMAX-28 transfer function to estimate past sea-surface temperatures. This regional SIMMAX method offers a slightly better understanding of the marginal sea conditions of the South China Sea than the linear transfer function FP-12E, which is based only on open-ocean data. However, both methods are biased toward the tropical temperature regime because of the very limited data from temperate to subpolar regions. The SIMMAX formula was applied to sediment core 17940 from the northeastern South China Sea, with sedimentation rates of 20-80 cm/ka. Results revealed nearly unchanged summer temperatures around 28°C for the last 30 ky, while winter temperatures varied between 19.5°C in the last glacial maximum and 26°C during the Holocene. During Termination 1A, the winter estimates show a Younger Dryas cooling by 3°C subsequent to a temperature optimum of 24°C during the Bölling=Alleröd. Estimates of winter temperature differences between 0 and 100 m water depth document the seasonal variations in the thickness of the mixed layer and provide a new proxy for estimating past changes in the strength of the winter monsoon.

(Table 2) Relative abundance of planktonic foraminifera species in Pliocene - Early Quaternary sediments from ODP Hole 111-677A

Planktonic foraminifera from Pliocene - Early Quaternary sediments of ODP Hole 111-677A were studied in detail. It was shown that the majority of detected zonal taxa are reliable biostratigraphic reference points. Between 30 and 210 m in the core zones of planktonic foraminifera from PL1b to Pt1 (according to the W.A. Berggren scale) were distinguished. Changes of planktonic foraminifera complexes from sediments of Hole 111-677A are closely associated with climate-controlled development of surface water masses of the Eastern Equatorial Pacific during 4.6-0.65 million years ago. Sharp decrease in equatorial-tropical species about 3.4 million years ago correlated with cessation of surface water exchange between tropical regions of the Pacific and Atlantic oceans due to formation of the Central American isthmus. The paleotemperature method of M.S Barash was used for reconstructing surface temperatures. Maximum temperatures were reconstructed in late Early Pliocene (26.4°C) and in Late Pliocene (26.6°C) and minimum ones - in the beginning of Early Pliocene (18.4°C), in the middle of Late Pliocene (19.6°C). Cold events occurred: 4.6-4.3, 2.8-2.5, and 1.7-1.2 million years ago, and warm: 4.3, 4.18-3.4, 2.5-2.3, and 1 million years ago. In general, the middle of Early Pliocene, the middle of late Pliocene and early Pleistocene are characterized by cold-water conditions, and the end of Early and the end of Late Pliocene - by warm-water conditions.