Abundance and size variation of Globorotalia menardii in the Northeastern Indian Ocean during the late quaternary

Variation in absolute abundance, size and coiling direction ratio of the planktic foraminifera Globorotalia menardii during the late Quaternary have been studied in three cores, collected along a north-south transect from the northeastern Indian Ocean. Increased abundance and larger size is observed during the warm interglacial period, whereas, smaller and fewer G. menardii specimens are noted during the cold glacial periods. A comparatively better match between absolute abundance, size and oxygen isotopic composition of foraminifera as compared to carbon isotopic composition at all the locations indicates that the abundance and size of G. menardii is more affected by seawater temperature and salinity as compared to productivity changes. The amplitude and frequency of temporal changes in the abundance and size of G. menardii vary from core-to-core, indicating the influence of site-specific physico-chemical conditions. In comparison to the abundance and size, the coiling direction of G. menardii does not show any systematic change in any of the cores. The majority of the specimens coil sinistrally. We conclude that the absolute abundance and variation in the average size of G. menardii can be used to infer past climatic changes. However, the variation in coiling direction of G. menardii appears to be unaffected by late Quaternary paleoclimatic variations in the north-eastern Indian Ocean.     

Surface ocean circulation in the Norwegian Sea 15,000 B.P. to present

Quantitative studies of foraminifera and radiolaria, semi-quantitative analyses of diatoms and coccoliths, and the distribution of ice-rafted sediments have been performed on cores from the southeastern Norwegian Sea. The results document large variations in sea-surface temperatures and ocean circulation, showing a strong correlation between oceanic data and palaeoclimatic data from the neighbouring coastal areas of Norway. For the first time the Allerød – Younger Dryas climatic fluctuations and the Holocene climatic optimum are shown in records from the Norwegian Sea. Starting at about 13,000 B.P. the sea surface became seasonally ice-free with productive seasons. During the Allerød a narrow wedge of temperate Atlantic water flowed into the southeastern Norwegian Sea. In Younger Dryas time the surface waters cooled by several degrees. Holocene surface conditions were relatively constant, with somewhat higher temperatures in a period possibly corresponding with Atlantic time.     

Fingerprinting the 8.2 ka event climate response in a coupled climate model

Using results from coupled climate model simulations of the 8.2 ka climate event that produced a cold period over Greenland in agreement with the reconstructed cooling from ice cores, we investigate the typical pattern of climate anomalies (fingerprint) to provide a framework for the interpretation of global proxy data for the 8.2 ka climate event. For this purpose we developed an analysis method that isolates the forced temperature response and provides information on spatial variations in magnitude, timing and duration that characterise the detectable climate event in proxy archives. Our analysis shows that delays in the temperature response to the freshwater forcing are present, mostly in the order of decades (30 a over central Greenland). The North Atlantic Ocean initially cools in response to the freshwater perturbation, followed in certain parts by a warm response. This delay, occurring more than 200 a after the freshwater pulse, hints at an overshoot in the recovery from the freshwater perturbation. The South Atlantic and the Southern Ocean show a warm response reflecting the bipolar seesaw effect. The duration of the simulated event varies for different areas, and the highest probability of recording the event in proxy archives is in the North Atlantic Ocean area north of 40° N. Our results may facilitate the interpretation of proxy archives recording the 8.2 ka event, as they show that timing and duration cannot be assumed to correspond with the timing and duration of the event as recorded in Greenland ice cores. Copyright © 2011 John Wiley & Sons, Ltd.     

Cretaceous Oceanic Red Beds: Distribution, Lithostratigraphy and Paleoenvironments

Cretaceous oceanic red beds (CORBs) represented by red shales and marls, were deposited during the Cretaceous and early Paleocene, predominantly in the Tethyan realm, in lower slope and abyssal basin environments. Detailed studies of CORBs are rare; therefore, we compiled CORBs data from deep sea ocean drilling cores and outcrops of Cretaceous rocks subaerially exposed in southern Europe, northwestern Germany, Asia and New Zealand. In the Tethyan realm, CORBs mainly consist of reddish or pink shales, limestones and marlstones. By contrast, marlstones and chalks are rare in deep-ocean drilling cores. Upper Cretaceous marine sediments in cores from the Atlantic Ocean are predominantly various shades of brown, reddish brown, yellowish brown and pale brown in color. A few red, pink, yellow and orange Cretaceous sediments are also present. The commonest age of CORBs is early Campanian to Maastrichtian, with the onset mostly of oxic deposition often after Oceanic Anoxic Events (OAEs), during the early Aptian, late Albian-early Turonian and Campanian. This suggests an indicated and previously not recognized relationship between OAEs, black shales deposition and CORBs. CORBs even though globally distributed, are most common in the North Atlantic and Tethyan realms, in low to mid latitudes of the northern hemisphere; in the South Atlantic and Indian Ocean in the mid to high latitudes of the southern hemisphere; and are less frequent in the central Pacific Ocean. Their widespread occurrence during the late Cretaceous might have been the result of establishing a connection for deep oceanic current circulation between the Pacific and the evolving connection between South and North Atlantic and changes in oceanic basins ventilation.     

Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230Th measurements

The cause of the climatically controlled fluctuations in the carbonate content of deep-sea sediments remains the subject of uncertainty and debate. Three variables are involved: supply of biogenic carbonate, loss by dissolution, and dilution by non-carbonate phases. It is suggested that ²³⁰Th, which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale. Results of a preliminary analysis based on published data indicate that, for depths at and above the lysocline, the carbonate fluctuations observed in cores from the North Atlantic Ocean are due primarily to variations in the terrigenous clay input, which was 2–5 times higher during glacials than during interglacials. Carbonate deposition appears to have been somewhat reduced during glacials, but probably not by more than a factor of 2. From published ${}^{230}\text{Th}{}^{232}\text{Th}$ profiles it appears that the South Atlantic Ocean also received increased inputs of terrigenous clay during glacial periods.     

Tree‐ring‐based annual precipitation reconstruction for the Hexi Corridor,NW China: consequences for climate history on and beyond the mid‐latitude Asian continent

An annual (July to June) precipitation reconstruction for the period AD 1760–2010 was developed from a Picea crassifolia regional tree‐ring chronology from two sites in the northern mountainous region of the Hexi Corridor, NW China. This reconstruction explains 52.1% of the actual precipitation variance during the period 1951 to 2010. Spatial correlations with gridded land‐surface data reveal that our reconstruction contains a strong regional precipitation signal for the Hexi Corridor and for the southern margin of the Badain Jaran Desert. Significant spectral peaks were identified at 31.9, 11.1, 8.0, 7.0, 3.2, 2.6 and 2.2 years. A large‐scale comparison indicates that our reconstruction is more consistent with climate records of a Westerly‐dominated Central Asia, and that the Westerlies have a greater impact on the precipitation in this region than the Asian summer monsoon. Our reconstructed precipitation series is significantly correlated with sea‐surface temperature (SST) in the tropical Atlantic Ocean (positive), the tropical Indian Ocean (positive), the western tropical Pacific Ocean (positive), and the western North Pacific Ocean (negative). The spatial correlation patterns between our precipitation reconstruction and SSTs of the Atlantic and Pacific Oceans suggest a connection between regional precipitation variations and the high‐mid‐latitude northern atmospheric circulations (Westerlies and Asian summer monsoon).     

Reconstruction of Postpliocene climatic changes by means of planktic Foraminifera

Six cores were selected from 46 collected in the SW part of the Atlantic Ocean to compare the three main methods of constructing Pleistocene paleoclimatic curves by means of planktic Foraminifera. It was found that the method based on a study of the whole fauna yields more details and represents the degree of the relative climatic changes better, whereas the method based on the ratio between the Globorotalia menardii complex and G. inflata exagerates the large scale temperature oscillations. However, this exaggeration makes the separation of glacial, interglacial, and interstadial epochs much easier. Paleoclimatic curves based on the sinistral: dextral ratio of Globorotalia truncatulinoides do not coinicde with those prepared utilizing the two previous methods. thus, the coiling direction of Globorotalia truncatulinoides cannot be utilized as a paleoclimatic criterion.     

Atlantic ocean heat piracy and the bipolar climate see‐saw during Heinrich and Dansgaard–Oeschger events

The millennial‐scale asynchrony of Antarctic and Greenland climate records during the last glacial period implies that the global climate system acts as a bipolar see‐saw driven by either high‐latitudinal and/or near‐equatorial sea‐surface perturbations. Based on the results of recent modelling of generic Heinrich and Dansgaard–Oeschger scenarios, we discuss the possibility that oscillations of the deep‐ocean conveyor may have been sufficient to cause this bipolar see‐saw. The bipolar climate asynchrony in our scenarios is caused by the toggle between North Atlantic heat piracy and South Atlantic counter heat piracy. Ocean circulation has an enhanced sensitivity to the northern deep‐water source as the North Atlantic Deep Water (NADW) cannot enter the Southern Ocean at depths shallower than the bottom of the Drake Passage. Any shoaling of the NADW can, therefore, increase the northward incursion of Antarctic Bottom Water (AABW), and trigger an interhemispheric climate oscillation. As hundreds of years are required to warm the respective high latitudes, the observed climate lead and lags between the two hemispheres can be explained entirely by the variability of the meridional overturning and by the corresponding change in the oceanic heat transport. Accordingly, it is entirely feasible for the global climate to work like a pendulum, which theoretically could be controlled by pushing at either of the deep‐water sources. Our model scenarios suggest that it is entirely feasible for the bipolar climate see‐saw to be controlled solely by variations in NADW formation. Copyright © 2001 John Wiley & Sons, Ltd.     

Reconstruction of palaeoceanographic conditions in the South Atlantic Ocean at the last two Terminations based on calcareous dinoflagellate cysts

 Despite the increasing interest in the South Atlantic Ocean as a key area of the heat exchange between the southern and the northern hemisphere, information about its palaeoceanographic conditions during transitions from glacial to interglacial stages, the so-called Terminations, are not well understood. Herein we attempt to increase this information by studying the calcareous dinoflagellate cysts and the shells of Thoracosphaera heimii (calcareous cysts) of five Late Quaternary South Atlantic Ocean cores. Extremely high accumulation rates of calcareous cysts at the Terminations might be due to a combined effect of increased cyst production and better preservation as result of calm, oligotrophic conditions in the upper water layers. Low relative abundance of Sphaerodinella albatrosiana compared with Sphaerodinella tuberosa in the Cape Basin may be the result of the relatively colder environmental conditions in this region compared with the equatorial Atlantic Ocean with high relative abundance of S. albatrosiana. Furthermore, the predominance of S. tuberosa during glacials and interglacials at the observed site of the western Atlantic Ocean reflects decreased salinity in the upper water layer. Received: 9 November 1998 / Accepted: 26 October 1999     

Late Quaternary changes in surface water and deep water masses of the Nordic Seas and north-eastern North Atlantic: a review

Quantitative and semiquantitative proxy data based on more than 200 core-top samples and 100 deep-sea cores lead to important new insights about late Quaternary changes in paleo-oceanography, climate and microfaunal habitats in the north-eastern North Atlantic and Nordic Seas, insights resulting from a detailed investigation by the Kiel research project SFB 313/132 summarized in this paper. Planktonic foraminifera species provide reliable tracers of past sea surface temperatures and currents. The genus Beella in particular was found to trace subtropical water masses up to the far north. Benthic foraminifera species served as sensors of bottom currents and local flux rates of organic matter. New orders of time resolution are reached via stable isotope stratigraphy and accelerator mass spectrometry carbon-14 dating, allowing the identification of meltwater events lasting a few hundred years and shorter, a time range where, however, the yet unquantified role of bioturbation presents a growing problem. Based on this high-resolution stratigraphy a number of time slices (synoptic time intervals) are defined to reconstruct the incursion of Atlantic water masses, to map paleocurrent patterns within the Nordic Seas and the north-eastern North Atlantic and to test alternative circulation models — for example, for the last glacial maximum (LGM) and various meltwater episodes. These are clearly coeval with Dansgaard-Oeschger events found in Greenland ice cores, with the actual cause of the flickering climate as yet unknown. Likewise, there is ongoing controversy about the extent of past sea-ice cover and about possible changes from the present anti-estuarine to estuarine mode of deep water exchange between the North Atlantic and the Nordic Seas during the LGM. South of Iceland, however, the history of deep water renewal over the last glacial cycle covering the last 30000 years was largely deciphered.     

Seasonal Dependence of the North Pacific and North Atlantic SST Predictability and Forecast Skill

In this paper, the decadal predictability and forecast skill of the Sea Surface Temperature Anomalies (SSTA) in the North Pacific and North Atlantic Ocean were investigated by conducting three sets of perfect model forecast experiments using a global coupled general circulation model. The results show that the annual mean SSTA in the North Pacific is less predictable on decadal time scale, with the forecast skill notably weaker than that of the North Atlantic. By analyzing the predictability and forecast skill of seasonal mean SSTA, it is found that the decadal predictability and forecast skill of the winter mean (JFM) SSTA in the central and western North Pacific are significantly higher than those of other seasons, and the magnitude is comparable with that of the North Atlantic. The predictability and forecast skill of the North Atlantic SSTA also show seasonal variations. Further analysis indicates that the seasonal dependence of the SSTA decadal predictability and forecast skill in the North Pacific is due to the winter-to-winter reemergence mechanism of SSTA in the North Pacific, which results from the seasonal variation of the mixed layer depth of the North Pacific Ocean. While the seasonal dependence of the North Atlantic SSTA predictability and forecast skill might be related to seasonal variations of other processes, such as the Atlantic Decadal Oscillation. The results of this paper suggest that for decadal climate prediction, if the forecast skill of the seasonal mean is taken into account, we might obtain higher than annual mean forecast skill for some seasons.     

Tropical sea-surface temperatures during the last glacial period: a view based on alkenones in Indian Ocean sediments

The qualitification of tropical temperatures during the last glacial cycle (0-150 kyr BP) is a controversial issue since different proxies seem to provide conflicting informations. To obtain a complementary point of view, we use the alkenone method to estimate sea-surface temperatures and focus our work on deep-sea sediments recovered from the tropical Indian Ocean. We present alkenone data obtained in two cores which cover in detail the last deglaciation and in about twenty cores distributed between 20°S and 20°N that were chosen to evaluate the temperature contrast of the last glacial-interglacial transition. Our results indicate that Indian Ocean tropical temperatures remained an average within 1.5-2.5°C of their present values during the last glaciation. At 10°N the last deglaciation is characterized by two warming steps which is similar to the classical deglacial chronology observed in the North Atlantic area. At 20°S the deglacial warming occurred at ca. 15 cal kyr BP, lagging significantly (5-4 kyr) behind the Antarctic warming, but in phase with northern hemisphere time series.     

Alkenones and coccoliths in ice‐rafted debris during the Last Glacial Maximum in the North Atlantic: implications for the use of UK37′ as a sea surface temperature proxy

The U^K₃₇′ index has proven to be a robust proxy to estimate past sea surface temperatures (SSTs) over a range of time scales, but like any other proxy, it has uncertainties. For instance, in reconstructions of the Last Glacial Maximum (LGM) in the northern North Atlantic, U^K₃₇′ indicates higher temperatures than those derived from foraminiferal proxies. Here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice‐rafted debris (IRD) to deep‐sea sediments. We have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern North Atlantic, and from a deep‐sea core from the Reykjanes Ridge. Certain debris flow deposits in the North Atlantic were generated by the presence of massive ice‐sheets in the past, and their associated ice streams. Such deposits are composed of the same materials that were present in the IRD at the time they were generated. We conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre‐Quaternary coccolith species during glacial stages, but that not all of the IRD contained alkenones, even when reworked coccoliths were present. We speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from IRD did occur at a particular site in the glacial North Atlantic. We also observe that alkenones in some of the debris flows contain a colder signal than estimated for LGM sediments in the northern North Atlantic. This is also clear in the deep‐sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or IRD. We conclude that any possible bias to U^K₃₇′ estimates associated with reworked alkenones is not necessarily towards higher values, and that the high SST anomalies for the LGM are unlikely to be the result of a bias caused by IRD inputs. Copyright © 2011 John Wiley & Sons, Ltd.     

The sandy channel-lobe depositional systems in the Gulf of Cadiz: Gravity processes forced by contour current processes

The sedimentation in the Gulf of Cadiz (NE Atlantic Ocean) is significantly controlled by the Mediterranean Outflow Water (MOW). Along its pathway onto the continental slope, the MOW is canalized by contourite channels, some of them feeding gravity sandy channel-lobe depositional systems firstly recognized in previous study [Habgood et al., 2003. Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50(3), 483-510.].Using very high resolution acoustic data and cores, a detailed characterization and a new evolution pattern of these channel-lobe depositional systems is established. Complex internal geometry of the lobes shows several depositional units revealing a polyphase evolution of these systems, with a general progradation punctuated by retrogradation and avulsion phases. A gravity origin controlled by contouritic processes and climatic changes is demonstrated for the feeding and the evolution of these sandy channel-lobe depositional systems. Climate oscillations, via the MOW variations, act as a major forcing of the activity of the channel-lobe depositional systems during the Late Quaternary.     

Regional features of primary alkaline magmas of the Atlantic Ocean

In the framework of the GIS project on the geochemistry of Atlantic intraplate magmatism, primary high-magnesian melts were identified there and subdivided into five types: foidites, picrites, basanite-nephelinites, alkaline olivine basalts, and tholeiite. Their relative proportions were determined for both the Atlantic Ocean as a whole and individual magmatic centers. The compositional ranges and average compositions were calculated. It was shown that alkali rocks are predominant, but tholeiite melts account for about 25%. Among ocean-island volcanic rocks, differentiated varieties clearly dominate over primary melts (80 and 20%, respectively). Variations in the proportions of the distinguished types were applied to prepare a map for the petrochemical typification of Atlantic intraplate magmatism. Seven petrochemical zones were provisionally identified, first demonstrating the lateral petrochemical heterogeneity of intraplate sources of the Atlantic Ocean. In addition to the global heterogeneity, each large center of intraplate magmatism (archipelago or island chain) demonstrates local heterogeneities. The variations in the Na/K, Ti/Na, and Si/Ca ratios reflect significant magma generation depths (in the lower mantle) for intraplate magmatism. It was proposed that variations in the Ti/Na ratio in the high-magnesian melts are controlled by a change in the Na and Ti partition coefficients of pyroxene with increasing magma generation depth. A comparison between evolution of the oceanic and continental alkaline magmatism was conducted.     

New data on the trace metal composition of the planktonic foraminifera microfossils of the Atlantic Ocean

This paper reports new data on the trace metal composition of planktonic foraminifer shells from surface sediments and cores (fraction >0.1 mm) in the central part of the Atlantic Ocean. This investigation has made it possible to identify a considerable accumulation of trace elements from water due to calcite entering into the crystal lattice under biomineralization and adsorption on the shell surface and pores, despite the fact that the shells are depleted in trace elements relative to pelagic clays. The trace element content in planktonic foraminifer microfossils is characterized by temporal variability, which is the most pronounced in long cores (Holocene–Upper Pleistocene) and reflects the sedimentation paleoenvironment in the ocean.     

Burial of redox-sensitive metals and organic matter in the equatorial Indian Ocean linked to precession

Authigenic metals (uranium, cadmium, and molybdenum), organic carbon (OC) and total C₃₇ alkenone (totC₃₇) concentrations were measured for the last 350 kyr in core MD900963, located in the eastern equatorial Arabian Sea. Authigenic metal concentrations on a carbonate-free basis range between 1 and 17 ppm, 0.5 and 6 ppm, and 0.5 and 4 ppm for U, Cd, and Mo, respectively. The profiles are characterized by well-defined 23 kyr cycles between oxic and mildly suboxic conditions. The redox-sensitive metal profiles also follow variations in the concentrations of OC (0.2-0.9%) and alkenones (0.2-6.7 ppm). The coupled variations in inorganic and organic constituents are attributed to a 23-kyr cycle in primary production above site MD900963, as suggested by clear correlations with independent micropaleontologic proxies (primary productivity indices based on foraminifera and coccoliths and fragmentation of foraminiferal shells). The 23-kyr cycles do appear to be primarily driven by productivity rather than changes in bottom water oxygen. Comparison with other records indicates that if this interpretation is correct, productivity variations across much of the Indian Ocean have been dominated by precessional forcing, with high productivity in phase with low summer insolation in the Northern Hemisphere. This interpretation contrasts with the traditional attribution of enhanced productivity in the Indian Ocean with periods of high summer insolation.     

New evidence on the sequence of deglacial warming in the tropical Indian Ocean

The transition from the Last Glacial Maximum to the Holocene was an internal of climate variability that was characterised by large spatial and temporal variations. Here we show that deglaciation warming in the northern Indian Ocean was initiated ca. 19 ka, which is contemporary with deglaciation warming in the Antarctica and Southern Ocean. A gradual warming occurred during the glacial/Holocene transition in the northern Indian Ocean, unlike the two‐step warming seen in Greenland and the North Atlantic. Synchronous deglacial warming ca. 19 ka in Antarctica and the northern Indian Ocean suggests a strong connection in the propagation of climate signals between Antarctica and the Indian Ocean, probably through the Indonesian Throughflow and/or Subantarctic Mode Water. Copyright © 2010 John Wiley & Sons, Ltd.     

利用树轮密度重建新疆北部5-8月温度变化

利用新疆北部5个采样点的树轮密度年表主序列和新疆北部33个气象站观测气温平均值进行相关分析,结果显示其与新疆北部5-8月平均温度存在很强一致相关性,相关系数为0.667（p<0.001, n=49）。温度重建方程能够解释校准期（1960-2008年）内44.5%的温度变化方差。空间相关分析揭示该温度重建序列能够表征新疆北部过去353 a （1656-2008年）5-8月温度总体变化特征。重建序列揭示新疆北部5-8月平均温度大致经历了6个偏暖阶段,即1656-1664年、1667-1692年、1711-1734年、1804-1832年、1855-1956年、2000-2008年,中间为偏冷阶段,这些阶段中间多个小幅度变化。温度重建结果发现新疆北部温度变化与太阳活动、火山喷发有着紧密联系。温度重建序列与全球海温场的相关分析显示当西风带海区及热带大西洋地区的海温偏高时,研究区气温偏高。与海温、火山喷发和云量变化的相关分析都指示在西风环流的作用下,上述因子对于该地区温度变化有着重要影响。该温度重建序列在低频变化上与北半球气温具有显著正相关,说明中亚地区温度变化与北半球整体温度变化具有较好的一致性。     

Book reviews

Books review in this article:
The Geology of the Atlantic Ocean, by K. O. Emery and Elazar Uchupi
Provenance of Arenites, ed. by G. G. Zuffa