East Antarctic Ice Sheet Expansion Research Articles

Miocene Sea Surface Dynamics in the Western Equatorial Pacific Based on Calcareous Nannofossil Records

Calcareous nannofossil in the sediments from Ocean Drilling Program (ODP) Hole 806B in the Ontong Java Plateau have been examined to reconstruct surface water condition in the western equatorial Pacific (WEP) throughout the Miocene. By using quantitative techniques, 107 sediment samples have been prepared into nannofossil slides and observed under microscope. Changes in Reticulofenestra size variations, Discoaster relative abundance and nannofossil accumulation rates (NAR) suggest that WEP surface water was dynamic. During early to middle Miocene from 18 to 14 Ma, the surface water was warm and depleted in nutrient with deep thermocline due to warm period of the Middle Miocene Climatic Optimum. Temperature cooled, nutrient increased and thermocline shoaled at 14 Ma during the East Antarctic Ice Sheet Expansion (EAIE). Surface water warmed and thermocline deepened after 13.2 Ma as the western Pacific warm pool (WPWP) began to established. At 9 Ma, surface water suddenly changed into cooler and nutrient rich condition. The collapse of ocean stratification coincided with the onset of Asian Monsoon Intensification. Warm surface water and deep thermocline occurred again in the latest Miocene as the brief return of the WPWP during 7–6 Ma.

Geochemistry and isotopic study of southern Bay of Bengal sediments: Implications for provenance and paleoenvironment during the middle Miocene

The Bay of Bengal (BoB), the largest bay in the world, is known to receive large amounts of fresh water and suspended particulate matter from different sources. The sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies. The Miocene Epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity, redox conditions and provenance of sediments. However, limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the Miocene. Here we present detailed geochemical and radiogenic (Sr, Nd) isotopic results from predominantly middle Miocene sediments of Ocean Drilling Program (ODP) Site 758A, located in the southern BoB (Lat. 5 °23′N, Long. 90°21′E; water depth 2925 m), to understand paleoredox conditions, paleoproductivity and provenance. The results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region.The distribution of redox-sensitive element proxies (U, V, Cr) at interval ~17–8 Ma points to an oxic depositional environment. The major, trace and rare earth elements distribution supports prevalence of the Indian summer monsoon (ISM) at ~12–10 Ma. Productivity proxies (Babio, Porg) show a decreasing trend during colder events (East Antarctic Ice Sheet expansion – EAIS and advent of late Miocene cooling – LMC events). The trace and rare earth element concentrations are generally low, except at intervals where glass shards were found. The 87Sr/86Sr and εNd(0) values indicate that the sediments belong to the High Himalayan sedimentary series (HHSS) throughout the middle Miocene. However, prior to initiation of the ISM, these sediments underwent meteoric alteration in the lowlands before deposition in the BoB. Our records suggest that sediments at ODP Site 758A may have been introduced into the BoB through the Irrawaddy-Yarlung-Tsangpo drainage system during the entire period of this study. The Irrawaddy-Yarlung-Tsangpo system drained the Indus-Tsangpo Suture zone, southern Tibet, the eastern Himalayas and the Arakan coast before debouching in the BoB.

Paleoceanography of the east equatorial Pacific over the past 16 Myr and Pacific–Atlantic comparison: High resolution benthic foraminiferal δ18O and δ13C records at IODP Site U1337

We present astronomically tuned high resolution (∼3–4 Kyr) 15.8-Myr long benthic foraminiferal δ18O and δ13C records from IODP Site U1337 in the east equatorial Pacific, which provide a reference standard for stratigraphic correlation of deep sea sediments. A long-term glaciation history over the past 15.8 Myr has been recorded in the δ18O. Two of the most significant glaciation events include the East Antarctic Ice Sheet expansion (EAIE) at 13.8 Ma and the final onset of the Northern Hemisphere Glaciation (NHG) in 3.3–2.5 Ma. A series of Miocene and Pliocene glaciation events exist between the EAIE and NHG, which are important glaciation attempts before the Earth finally entered the full “icehouse” world in the late Pleistocene. A long-term ocean carbon isotope decrease over the past 15.8 Myr has been recorded in the δ13C. Ocean circulation change was the major cause of the ocean carbon isotope decrease in 9.4–8.1 Ma and 4–0 Ma, whereas strengthened continental chemical weathering controlled the long-term ocean carbon isotope decrease in most of the time of the past 15.8 Myr. The divergence in benthic foraminiferal δ13C and εNd provides strong evidence of significant ocean circulation change between 12.1 and 9.2 Ma which was probably related to the formation of the Isthmus of Panama. The internal feedbacks such as continental weathering and sea surface temperature change have great potential for modulating the weak 100 and 400 Kyr signals in astronomical forcing into strong 100 and 400 Kyr signals in global ice volume and carbon cycle.

Changes of deep Pacific overturning circulation and carbonate chemistry during middle Miocene East Antarctic ice sheet expansion

East Antarctic ice sheet expansion (EAIE) at ∼13.9 Ma in the middle Miocene represents a major climatic event during the long-term Cenozoic cooling, but ocean circulation and carbon cycle changes during this event remain unclear. Here, we present new fish teeth isotope (εNd) and benthic foraminiferal B/Ca records from the South China Sea (SCS), newly integrated meridional Pacific benthic foraminiferal δ18O and δ13C records and simulated results from a biogeochemical box model to explore the responses of deep Pacific Ocean circulation and carbon cycle across EAIE. The εNd and meridional benthic δ13C records reveal a more isolated Pacific Deep Water (PDW) and a sluggish Pacific meridional overturning circulation during the post-EAIE with respect to the pre-EAIE owing to weakened southern-sourced deep water formation. The deep-water [CO2−3] and calcium carbonate mass accumulation rate in the SCS display markedly similar increases followed by recoveries to the pre-EAIE level during EAIE, which were probably caused by a shelf–basin shift of CaCO3 deposition and strengthened weathering due to a sea level fall within EAIE. The model results show that the ∼1‰ positive δ13C excursion during EAIE could be attributed to increased weathering of high-δ13C shelf carbonates and a terrestrial carbon reservoir expansion. The drawdown of atmospheric CO2 over the middle Miocene were probably caused by combined effects of increased shelf carbonate weathering, expanded land biosphere carbon storage and a sluggish deep Pacific meridional overturning circulation.

Vertical thermal gradient history in the eastern equatorial Pacific during the early to middle Miocene: Implications for the equatorial thermocline development

AbstractKnowledge of the equatorial thermocline is essential for understanding climate changes in the tropical Pacific. Multispecies planktic foraminiferal analyses provide a way to examine temperature distributions and thus the structure of the thermocline. Although the secular thermocline development has been documented back to the late Miocene, the early to middle Miocene interval has rarely been examined. In addition, relationships with the dynamic Antarctic ice sheets remain unclear. Here we investigate the vertical thermal gradient in the upper water column at Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific (EEP) throughout the early to middle Miocene (23.1 to 11.7 Ma). The gradient increased over the Miocene Climatic Optimum, whereas it decreased during the East Antarctic Ice Sheet Expansion (EAIE). Comparison of the EEP record with its western equatorial Pacific (WEP) counterpart suggests that sea surface temperature was more stable in the WEP than in the EEP. We further estimated equatorial thermocline from two diagonal gradients between the EEP and the WEP: thermocline shoaled from 16.7 to 15.7 Ma and tilt weakened between 16.5 and 13.8 Ma. The onset of the “Monterey Excursion” and the reduced Antarctic ice sheet volume would have affected thermocline depth and tilt, respectively. Thermocline depth was likely much deeper compared to Pliocene‐to‐modern conditions. Furthermore, a 4‐point‐based distribution of isotherms (4DI index) was used as a metric of the evenness or unevenness of the isotherm distributions. The 4DI index considerably reduced at around the EAIE and other Mi‐events, reflecting the evenly distributed isotherms under a more glaciated Antarctica.

Eastern equatorial Pacific benthic foraminiferal distribution and deep water temperature changes during the early to middle Miocene

Abstract We investigate changes in benthic foraminiferal assemblages and estimate bottom water temperatures (BWT), based on Mg/Ca ratios of Oridorsalis umbonatus , over three selected intervals of major climate change during the late early to middle Miocene at Integrated Ocean Drilling Program (IODP) Sites U1337 (4463 m water depth) and U1338 (4200 m water depth), eastern equatorial Pacific Ocean. The targeted intervals are: (1) the onset of the Miocene Climatic Optimum (MCO; at ~ 16.9 Ma) and associated carbon cycle perturbation (Site U1337); (2) the episode of peak warmth within the MCO centered at 15.6 Ma (Site U1338); (3) the major cooling step at ~ 13.8 Ma related to east Antarctic ice-sheet expansion (Site U1338). Assemblages from these three intervals are mainly composed of cosmopolitan, long ranging (e.g., Paleogene to recent), lower bathyal to abyssal species, including calcareous elongated forms that became extinct during the middle Pleistocene. The onset of the MCO and interval of peak warmth had little impact on benthic foraminiferal accumulation rates (BFAR), abundances and assemblage composition. A transient BWT warming of 2.6 °C at the onset of the interval of peak warmth concurred with a benthic stable oxygen isotope (δ 18 O) decrease of 1‰, suggesting that the δ 18 O decrease was largely controlled by BWT changes. A substantial increase in BFAR and in the abundances of Epistominella exigua and epifaunal taxa after ~ 13.83 Ma coincides with increased opal accumulation rates, reflecting enhanced and seasonally pulsed organic carbon (C org ) flux to the seafloor during the global cooling step. During this interval, BWT decrease by 1.7 °C concurrent with a 1.0‰ increase in the stable oxygen isotope composition of seawater (δ 18 O sw ). Comparable trends at Ocean Drilling Program (ODP) Site 1146 in the South China Sea, with a 2.2 °C cooling and 0.9‰ increase in δ 18 O sw at ~ 13.8 Ma, further support that the benthic δ 18 O increase during the major cooling step was largely controlled by ice volume changes.

Obliquity and long eccentricity pacing of the Middle Miocene climate transition

AbstractThe Middle Miocene East Antarctic ice sheet expansion (EAIE), which is indicated by an abrupt ~1‰ increase in global benthic foraminiferal δ18O at ~13.8 Ma, marks the Middle Miocene climate transition (MMCT) and has been related to astronomically modulated changes in the global carbon cycle. Here, we present high resolution (3–4 kyr) benthic foraminiferal δ18O and δ13C records from IODP Site U1337 in the central equatorial Pacific, which spans the period 12.2–15.8 Ma. The isotopic records clearly demonstrate significant imprints from periodic variations in the Earth's orbital parameters, particularly the obliquity (40 kyr) and the long eccentricity (400 kyr) cycles. While the benthic δ18O and δ13C exhibit nearly identical amplitudes for glacial‐interglacial cycles from 15.8 to 12.2 Ma, the long‐term trends in the benthic δ18O and δ13C had started to reverse after the beginning of the EAIE. Within the 400‐kyr band, the benthic −δ18O and δ13C displays a constant phase relationship between 15.8 and 12.2 Ma. At the 41‐kyr band, however, a phase reversal reaching >180° between −δ18O and δ13C occurs from 13.8 Ma to 14.0 Ma during the period of the EAIE. A similar phase relationship of benthic foraminiferal −δ18O and δ13C at the 400‐kyr band and the 41‐kyr band is also observed at ODP Site 1146 from the northern South China Sea. This phase jump occurs when the long‐term trends in δ18O and δ13C split, suggesting a decoupling of the global ice volume and ocean carbon reservoir changes during the Middle Miocene.

Deep-sea pre-glacial to glacial sedimentation in the Weddell Sea and southern Scotia Sea from a cross-basin seismic transect

Abstract Identification of the pre-glacial, transitional and full glacial components in the deep-sea sedimentary record is necessary to understand the ice sheet development of Antarctica and to build circum-Antarctic sediment thickness grids for palaeotopography/-bathymetry reconstructions, which constrain palaeoclimate models. A ~ 3300 km long Weddell Sea to Scotia Sea multichannel seismic reflection data transect was constructed to define the first basin-wide seismostratigraphy and to identify the pre-glacial to glacial components. Seven main seismic units were mapped: Of these, WS-S1, WS-S2 and WS-S3 comprise the inferred Cretaceous–Palaeocene pre-glacial regime (> 27 Ma in our age model), WS-S4 the Eocene–Oligocene transitional regime (27–11 Ma) and WS-S5, WS-S6, WS-S7 the Miocene–Pleistocene full glacial climate regime (11–1 Ma). Sparse borehole data from ODP Leg 113 and SHALDRIL constrain the ages of the upper three seismic units and seafloor spreading magnetic anomalies compiled from literature constrain the basement ages in the presented age model. The new horizons and stratigraphy often contradict local studies and show an increase in age from southeast to the northwest. The up to 1130 m thick pre-glacial seismic units form a mound in the central Weddell Sea basin and in conjunction with the eroded flank geometry, allow the interpretation of a Cretaceous proto-Weddell Gyre bottom current. The base reflector of the transitional seismic unit has a model age of 26.6–15.5 Ma from southeast to northwest, suggesting similar southeast to northwest initial ice sheet propagation to the outer shelf. We interpret an Eocene East Antarctic Ice Sheet expansion, Oligocene grounding of the West Antarctic Ice Sheet and Early Miocene grounding of the Antarctic Peninsula Ice Sheet. The transitional regime sedimentation rates in the central and northwestern Weddell Sea (6–10 cm/ky) are higher than in the pre-glacial (1–3 cm/ky) and full glacial regimes (4–8 cm/ky). The pre-glacial to glacial rates are highest in the Jane- and Powell Basins (10–12 cm/ky). Total sediment volume in the Weddell Sea deep-sea basin is estimated at 3.3–3.9 × 106 km3.

Middle Miocene seasonal temperature changes in the Styrian basin, Austria, as recorded by the isotopic composition of pectinid and brachiopod shells

Using the isotope record on pectinid and brachiopod shells, we reconstructed Middle Miocene palaeotemperatures for a shelf environment in the Styrian basin, Austria. Bivalve shells of Macrochlamys sp., collected from different horizons of the Lower Badenian Leitha limestone, show δ 18O values in the range of −3 to 0‰ (PeeDee Belemnite). The large intrashell variability of 3‰ indicates significant seawater temperature fluctuation. The results suggest that despite the high geographical latitude (ca. 40°N), the climate in southern Austria was warm (subtropical) with a pronounced seasonality at that time. In contrast, the δ 18O isotopic composition of a pectinid ( Flabellipecten sp.) and an indeterminate terebratulid brachiopod, from the siliciclastic deposits overlying the Leitha limestone, ranges between 0 and −1‰. This indicates cooler mean annual temperatures and reduced seasonal variations. A 39Ar/ 40Ar age on fresh volcanic biotites shows a value of 14.2±0.1 Ma. The tuffs containing the biotites are located below the layer containing the terebratulid and the Flabellipecten sp. The stable isotope and radiogenic data suggest that the drop in temperature is in direct relationship with the East Antarctic Ice Sheet expansion, which started at ca. 14 Ma. Due to the variation in temperature, the subtropical fauna in the Retznei quarry disappeared during the Early Miocene and the biogenic limestones were replaced by clastic sediments. The δ 18O isotopic profiles from three pectinids collected from the Leitha limestone indicate that these grew within ca. 1.5 years. Growth interruptions occurred during the warm season because during this period the pectinids very likely used their energy for the growth and maturation of gonads. Carbon isotopic compositions vary between 0 and 2‰. Statistical tests show that for some of the analyzed shells, the cyclicity of the δ 13C profiles may be explained by the temperature-dependent fractionation between air CO 2 and dissolved bicarbonate.

Were West Antarctic Ice Sheet grounding events in the Ross Sea a consequence of East Antarctic Ice Sheet expansion during the middle Miocene?

Seismic correlation of glacial unconformities from the Ross Sea outer continental shelf to chronostratigraphic control at DSDP sites 272 and 273 indicates that at least two West Antarctic Ice Sheet (WAIS) expansions occurred during the early part of the middle Miocene (i.e. well before completion of continental-scale expansion of the East Antarctic Ice Sheet (EAIS) inferred from δ 18O and eustatic shifts). Therefore, if the volume of the EAIS was indeed relatively low, and if the Ross Sea age model is valid, then these WAIS expansions/contractions were not a direct consequence of EAIS expansion over the Transantarctic Mountains onto West Antarctica. An in-situ development of the WAIS during the middle Miocene suggests that either West Antarctic land elevations were above sea level and/or that air and water temperatures were sufficiently cold to support a marine-based ice sheet. Additional chronostratigraphic and lithologic data are needed from Antarctic margins to test these speculations.

The stratigraphy of the Pliocene—lower Pleistocene Bardin Bluffs Formation, Amery Oasis, northern Prince Charles Mountains, Antarctica

In the Amery Oasis of the northern Prince Charles Mountains, the glaciomarine Bardin Bluffs Formation of the Pagodroma Group was deposited between the Late Pliocene (<3.1 Ma) and Early Pleistocene (>1 Ma). The formation provides evidence of (i) a reduced East Antarctic ice sheet compared to that of the present day and (ii) a subsequent Plio–Pleistocene ice sheet expansion. The formation consists of two members. The older, basal Member 1 is c. 12.5 m thick and consists of relatively ice-distal silty, sandy and sparsely fossiliferous fjordal strata. Member 1 reflects largely ice-free marine sedimentation c. 250 km inland from the current Amery Ice Shelf edge. The member is restricted to the area about the north-eastern end of Pagodroma Gorge where it infills a chemically weathered erosion surface, cut in the form of a valley on the Permo-Triassic Amery Group. Weathering occurred during aerial exposure of the Amery Oasis in a warmer climate than that of today. The younger Member 2 exceeds 40 m in thickness and is made up of coarse ice proximal glaciomarine diamicts. It overlies disconformably Member 1 at Pagodroma Gorge. Elsewhere, Member 2 rests directly upon a smoothed and striated erosion surface, cut on the Amery Group, which was part of a fjord floor. This erosional surface and the facies contrast between the two members, indicates an East Antarctic Ice Sheet expansion and Lambert Glacier grounding-line advance.

Variations in Miocene phytoplankton growth rates in the southwest Atlantic: Evidence for changes in ocean circulation

Changes in ocean circulation are often credited as the primary control on large‐scale climate change during the Miocene. This study investigates the latest Oligocene to middle Miocene evolution of Southern Ocean circulation by evaluating stable isotopic trends of shallow‐ and deep‐dwelling planktonic foraminifera, as well as εp records reconstructed from the carbon isotopic composition of diunsaturated alkenones in the southwestern Altantic Ocean (Deep Sea Drilling Project site 516). Changes in εp at site 516 closely paralleled the opening and deepening of the Drake Passage as inferred from seafloor magnetic anomalies. A large negative shift in εp at ∼20.3 Ma is interpreted to reflect an increase in upper water column nutrient concentrations, caused by the onset or strengthening of the Antarctic Circumpolar Current (ACC). Measurable alkenone concentrations disappear by ∼17 Ma, prior to a collapse in surface‐to‐thermocline δ18O and δ13C gradients. This is interpreted as reflecting a severe decrease in mixed layer nutrient concentrations and reduced proto‐Antarctic Intermediate Water influence. The δ18O gradient was reestablished by 14.5 Ma, coincident with the hypothesized East Antarctic ice sheet expansion, suggesting a direct relationship between increased strength of the ACC and the largest climate shift of the middle Miocene.