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ABSTRACT: The processes causing the middle Miocene global cooling, which marked the Earth's final transition into an 'icehouse' climate about 13.9 million years ago (Myr ago), remain enigmatic. Tectonically driven circulation changes and variations in atmospheric carbon dioxide levels have been suggested as driving mechanisms, but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult. Here we present high-resolution climate proxy records, covering the period from 14.7 to 12.7 million years ago, from two complete sediment cores from the northwest and southeast subtropical Pacific Ocean. Using new chronologies through the correlation to the latest orbital model, we find relatively constant, low summer insolation over Antarctica coincident with declining atmospheric carbon dioxide levels at the time of Antarctic ice-sheet expansion and global cooling, suggesting a causal link. We surmise that the thermal isolation of Antarctica played a role in providing sustained long-term climatic boundary conditions propitious for ice-sheet formation. Our data document that Antarctic glaciation was rapid, taking place within two obliquity cycles, and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change.
Nature 12/2005; 438(7067):483-7. · 36.28 Impact Factor
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ABSTRACT: We present centennial records of sea surface and upper thermocline temperatures in Core MD01-2378 from the Timor Sea, which provide new insights into the variability of the Indonesian outflow across the last two glacial terminations. Mg/Ca in Globigerinoides ruber (white s. s.) indicates an overall increase of 3.2 °C in sea surface temperature (SST) over Termination I. Following an early Holocene plateau at 11.3–6.4 ka, SSTs cooled by 0.6 °C during the middle to late Holocene (6.4–0.7 ka). The early Holocene warming occurred in phase with increasing northern hemisphere summer insolation, coinciding with northward displacement of the Intertropical Convergence Zone, enhanced boreal summer monsoon and expansion of the Indo-Pacific Warm Pool. Thermocline temperatures (Pulleniatina obliquiloculata Mg/Ca) gradually decreased from 24.5 to 21.5 °C since 10.3 ka, reflecting intensification of a cool thermocline throughflow. The vertical structure of the upper ocean in the Timor Sea evolved in similar fashion during the Holocene and MIS5e, although the duration of SST plateaux differed (11.3 to 6.4 ka in Termination I and from 129 to 119 ka in Termination II), which was probably due to the more intense northern hemisphere summer insolation during MIS 5e. During both terminations, SST increased simultaneously in the southern high latitudes and the tropical eastern Indian Ocean, suggesting virtually instantaneous atmospheric climate feedbacks between the high and low latitudes.
Earth and Planetary Science Letters.
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ABSTRACT: One of the most enigmatic features of Cenozoic long-term climate evolution is the long-lasting positive carbon-isotope excursion or “Monterey Excursion”, which started during a period of global warmth after 16.9 Ma and ended at ∼ 13.5 Ma, approximately 400 kyr after major expansion of the Antarctic ice-sheet. We present high-resolution (1–9 kyr) astronomically-tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern Pacific (ODP Sites 1146 and 1237), which shed new light on the middle Miocene carbon-isotope excursion and associated climatic transition over the interval 17.1–12.7 Ma. We recognize three distinct climate phases with different imprints of orbital variations into the climatic signals (1146 and 1237 δ18O, δ13C; 1237 XRF Fe, fraction > 63 μm): (1) climate optimum prior to 14.7 Ma characterized by minimum ice volume and prominent 100 and 400 kyr variability, (2) long-term cooling from 14.7 to 13.9 Ma, principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced δ13C increase, (3) “Icehouse” mode after 13.9 Ma with distinct 100 kyr variability and improved ventilation of the deep Pacific. The “Monterey” carbon-isotope excursion (16.9–13.5 Ma) consists overall of nine 400 kyr cycles, which show high coherence with the long eccentricity period. Superposed on these low-frequency oscillations are high-frequency variations (100 kyr), which closely track the amplitude modulation of the short eccentricity period. In contrast to δ13C, the δ18O signal additionally shows significant power in the 41 kyr band, and the 1.2 Myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 δ18O signal. Our results suggest that eccentricity was a prime pacemaker of middle Miocene climate evolution through the modulation of long-term carbon budgets and that obliquity-paced changes in high-latitude seasonality favored the transition into the “Icehouse” climate.
Earth and Planetary Science Letters.
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ABSTRACT: Recent drilling on the Kerguelen Plateau (Ocean Drilling Program Leg 183) has provided a unique and exciting high latitude record of palaeoceanographic change during the Cenomanian–Turonian in the Southern Ocean. The benthic foraminiferal succession at Site 1138 records the evolution of the Kerguelen Plateau from a subaerially exposed platform in the Cenomanian to a bathyal, pelagic environment in the early Turonian, following a major transgressive pulse and increased thermal subsidence of the Kerguelen Plateau, which led to a sea-level rise of possibly several hundred metres. Diversified benthic foraminiferal assemblages indicate an upper bathyal, mesotrophic setting after the peak of the transgression. The assemblages exhibit strong similarities to temperate, shelf and slope assemblages in the Northern Hemisphere. This bimodal distribution reflects the existence of open oceanic gateways and a dynamic trans-hemispheric global circulation. Equatorial assemblages are characterized by a low-diversity, high carbon flux biofacies. Assemblages from Alaska demonstrate high organic productivity and low oxygen conditions and the prevalence of elevated temperatures on the flooded shelf of the North Slope. Our results show that the distribution of upper bathyal benthic foraminifera was strongly modulated by carbon flux and oxygenation fluctuations, and not by physical migration barriers.
Cretaceous Research.
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ABSTRACT: We present a high-resolution (∼ 60–110 yr) multi-proxy record spanning Marine Isotope Stage 3 from IMAGES Core MD01-2378 (13°04.95′ S and 121°47.27′ E, 1783 m water depth), located in the Timor Sea, off NW Australia. Today, this area is influenced by the Intertropical Convergence Zone, which drives monsoonal winds during austral summer and by the main outflow of the Indonesian Throughflow, which represents a key component of the global thermohaline circulation system. Thus, this core is ideally situated to monitor the linkages between tropical and high latitude climate variability. Benthic δ18O data (Planulina wuellerstorfi) clearly reflect Antarctic warm events (A1–A4) as recorded by the EPICA Byrd and Dronning Maud Land ice cores. This southern high latitude signal is transferred by deep and intermediate water masses flowing northward from the Southern Ocean into the Indian Ocean. Planktonic δ18O shows closer affinity to northern high latitudes planktonic and ice core records, although only the longer-lasting Dansgaard–Oeschger warm events, 8, 12, 14, and 16–17 are clearly expressed in our record. This northern high latitude signal in the surface water is probably transmitted through atmospheric teleconnections and coupling of the Asian–Australian monsoon systems. Benthic foraminiferal census counts suggest a coupling of Antarctic cooling with carbon flux patterns in the Timor Sea. We relate increasing abundances of carbon-flux sensitive species at 38–45 ka to the northeastward migration of the West Australian Current frontal area. This water mass reorganization is also supported by concurrent decreases in Mg/Ca and planktonic δ18O values (Globigerinoides ruber white).
Marine Micropaleontology.
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ABSTRACT: The benthic stable isotope record from ODP Site 761 (Wombat Plateau, NW Australia, 2179.3 m water depth) documents complete recovery of the middle Miocene δ13C excursion corresponding to the climatic optimum and subsequent expansion of the East Antarctic Ice Sheet. The six main δ13C maxima of the “Monterey Excursion” between 16.4 and 13.6 Ma and the characteristic stepped increase in δ18O between 14.5 and 13.9 Ma are clearly identified. The sedimentary record of the shallower ODP Sites 1126 and 1134 [Great Australian Bight (GAB), SW Australia, 783.8 and 701 m water depth, respectively] is truncated by several unconformities. However, a composite benthic stable isotope curve for these sites provides a first middle Miocene bathyal record for southwest Australia. The δ18O and δ13C curves for Sites 1126 and 1134 indicate a cooler, better-ventilated water mass at ∼700 m water depth in the Great Australian Bight since approximately 16 Ma. This cooler and younger water mass probably originated from a close southern source. Cooling of the bottom water at ∼16 Ma started much earlier than at other sites of equivalent paleodepths in the central and western parts of the Indian Ocean. At Site 761, the δ18O curve shows an excellent match with the global sea level curve between ∼11.5 and 15.1 Ma, and thus closely reflects changes in global ice volume. Prior to 15.1 Ma, the mismatch between the δ18O curve and the sea level curve indicates that δ18O fluctuations are mainly due to changes in bottom water temperature.
Palaeogeography, Palaeoclimatology, Palaeoecology.
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ABSTRACT: The “Mohammed Plage” coastal section and well S13 in the Tarfaya Basin (southern Morocco) are key sections to investigate the subsidence of the basin and the Cenomanian eustatic sea level history. Results from a canonical correspondence analysis (CCA), based on benthic foraminiferal assemblage counts and additional proxies (percent planktic foraminifera, planktic foraminifera morphogroups, total organic carbon content (TOC)) show a general deepening trend for the investigated sections. The most important factor identified in the ordination of species and samples is water depth. The CCA identifies Bolivina anambra, Globulina lacrima, Lenticulina spissocostata and Spiroplactammina sp. as typical “shallow-water species”, Praebulimina nannina, Gavelinella dakotensis, Gavelinella sp., Saccammina alexanderi and Valvulineria lenticula as “deep-water species”, and Gabonita levis, Gabonita obesa, Neobulimina albertensis, Lingulogavelinella asterigerinoides, Lenticulina gaultina, Haplophragmoides gigas, Haplophragmoides bauchensis and Trochammina taylorana as unrestricted or intermediate species. The temporal succession of benthic foraminiferal assemblages indicates repeated periods of shallowing within the general deepening trend, in particular within the lower portion of the sections (Rotalipora brotzeni and R. reicheli planktic foraminiferal zones). Restricted assemblages occurred when intensification of the oxygen minimum zone inhibited a diverse benthic community. Abundant high organic flux indicators at the base of the “Mohammed Plage” section indicate that an upwelling cell off northwest Africa was already active during the early Cenomanian.
Marine Micropaleontology.
