Ocean Drilling Program Site Research Articles

Spatiotemporal Characterization of Smectite‐to‐Illite Diagenesis in the Nankai Trough Accretionary Prism Revealed by Samples From 3 km Below Seafloor

AbstractThe up‐dip limit of seismogenesis in subduction zone forearcs depends on the lithological composition of the incoming sediment and its subsequent modification during compaction and diagenesis. Here we present results of a multimethodological approach to characterize the smectite‐to‐illite diagenesis in the accretionary prism of the Nankai Trough subduction zone offshore SW Japan. Our X‐ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy clay mineralogical analysis reveal that advanced states of smectite‐to‐illlite diagenesis occur in samples recovered from down to 3 km subseafloor at Integrated Ocean Drilling Program Site C0002, which sampled the inner accretionary prism. Our temperature‐ and time‐dependent reaction kinetics models require elevated temperatures in the prism to explain the illitization, which is consistent with revised thermal models based on recent plate reconstructions. Biostratigraphic data suggest that the inner prism sediment was deposited during a period of slow or inactive subduction and buried in the accretionary prism after the Philippine Sea plate resumed subduction. Rapid burial in the past 6 Ma led to the formation of two authigenic smectite‐illite phases consistent with a broad 1–9.3 Ma age determined by K‐Ar dating. The low K+ smectite‐illite reflects the long‐term burial history, whereas the K+‐rich smectite‐illite was inherited from the younger accretion event. Our study predicts the illitization of smectite approaches completion at ~5 km below the seafloor in the hanging wall of the plate boundary and the megasplay fault zone, respectively, which coincides with the proposed seaward extent of coseismic slip in the 1944 Mw = 8.1 Tonankai earthquake along those faults.

Cyclostratigraphy of the Late Miocene to Pliocene sediments at IODP sites U1425 and U1430 in the Japan Sea and paleoceanographic implications

High-resolution studies of sedimentary sequences have reconstructed paleoceanographic changes in the Atlantic and southern and equatorial Pacific Oceans during the Late Miocene and Pliocene, but comparable analyses are lacking for the North Pacific Ocean. However, continuous samples of hemipelagic sequences covering this time interval were obtained at Integrated Ocean Drilling Program sites U1425 and U1430 in the Japan Sea during expedition 346. Because the paleoceanography of the Japan Sea was sensitive to glacio-eustatic sea-level changes, changes in the sediment record are manifested as cyclic lithological changes. By using a gamma ray attenuation (GRA) density, which reflects biogenic silica content, as an indicator of sea-level changes, we developed an independent orbitally tuned chronology at each site to construct high-resolution, high-precision age models for the time span of 11.8 to 1.45 Ma. First, the 405-kyr-filtered GRA profile at each site was tuned to 405-kyr-filtered orbital eccentricity. Then, using the 405-kyr-tuned age model, the 100-kyr-filtered GRA profile was tuned to short-eccentricity cycles to adjust the 405-kyr-tuned age models. We used the resulting age models to revise the time intervals of previously published lithological units and paleoceanographic stages for the Japan Sea. Our profiles based on physical properties are a good match to the oxygen isotope record, except for 6.6–3.6 Ma, a time interval with small-amplitude fluctuations in the oxygen isotope record.

Trace fossils evidence of a complex history of nutrient availability and oxygen conditions during Heinrich Event 1

Abstract The relationship between the behavior of tracemakers and paleoenvironmental conditions has been shown to be a useful tool in paleoceanographic studies. Here we present a detailed ichnological analysis of Heinrich Event 1 (H1), the youngest glacial North Atlantic large iceberg discharge, from Integrated Ocean Drilling Program Site U1308 in the central North Atlantic, located 2800 km from the mouth of the Hudson Strait. The ichnoassemblage consists of small trace fossils such as Phycosiphon, “Mycellia”/Trichichnus and Chondrites, and large ones including Planolites and Thalassinoides. The bioturbation index is generally low, from no bioturbation, to 30% of total bioturbated surface. Significant variations in composition, diversity and abundance of trace fossils are recorded, associated with the dual nature of H1 (comprising H1.1 and H1.2) and the period intervening between the two. Bottom- and pore-water oxygenation and benthic food availability are found to be the main limiting conditions for the tracemaker community, with some variations during the time span of H1, while factors such as sea water temperature had a minor impact.

Reappraisal of Oligocene-Miocene chronostratigraphy and the Mi-1 event: Ocean Drilling Program Site 744, Kerguelen Plateau, southern Indian Ocean

The Oligocene-Miocene Transition (OMT) is an important interval in Earth's history, with substantial changes in Antarctic ice volume and mean temperature. The OMTis complicated in paleoclimatology, chronostratigraphy, and terminology, and there is a need for additional sites outside of the tropic/subtropics or Atlantic Basin to better understand the cause(s) and consequences of the OMT. Ocean Drilling Program Hole 744A sits on the southern edge of the Kerguelen Plateau, and is thus a sensitive recorder of changes on Antarctica and in the Indian Ocean sector of the Southern Ocean. Previous stratigraphic studies have concluded that the Oligocene/Miocene boundary is contained within a short coring gap (1 m) between Cores 744A-11 and 12. A new benthic foraminiferal stable isotope curve is at odds with the existing age model for the upper ~0.5mof Core 12. A number of different explanations for age models are discussed, and we find there is only one plausible explanation for the chronostratigraphy through Core 12. We suggest therewas a short (~150-200 kyr) hiatus, encompassing the peak Mi-1 excursion, based on interpreted magnetostratigraphy. This hiatus has no obvious lithologic expression.

Quaternary sedimentary record in the northern Okinawa Trough indicates the tectonic control on depositional environment change

During the Quaternary, global sea level was characterized by large fluctuations in amplitude and increased frequency. However, the eastern marginal seas of China, especially the northern East China Sea and Yellow Sea did not experience large-scale transgression until the middle Pleistocene. The Zhe-Min (Zhejiang-Fujian) Uplift was an important barrier in the East China Sea preventing transgression of sea water into the northern marginal seas of China, but its Quaternary evolution is still poorly constrained. Here we present a thick sand layer record (the top age is ~416 ka) of the Integrated Ocean Drilling Program (IODP) Site U1428 from the northern Okinawa Trough to reconstruct the history of the Zhe-Min Uplift since the middle Pleistocene. Provenance analysis indicates that sediments from both sand and imbedded fine-grained calcareous-rich clay layers at the study site were mainly derived from the Yellow River, which for the first time indicates that the Yellow River sediments could influence the northern Okinawa Trough since at least the middle Pleistocene. Significant change of lithology at IODP Site U1428 at ~416 ka was attributed to the large-scale subsidence of the Zhe-Min Uplift, which is probably due to the rapid uplifting of Tibet during 0.52–0.38 Ma. Before ~416 ka, obstacle of the emersed Zhe-Min Uplift induced extremely severe reworking and promoted sand ridge formation in the shelf edge of southeastern Yellow Sea, and supplied sand sediments to the northern Okinawa Trough during transgression and high sea-level stages. After the subsidence of the Zhe-Min Uplift at ~416 ka, westward of tidal sand ridge and Yellow River mouth with the process of shelf transgressions favored fine-grained sediments input to the northern Okinawa Trough. Even during low sea-level stages, the flat terrain of the East Asian marginal sea shelf and low rainfall amount resulted in low discharge of Yellow River, and supplied fine-grained particles to the northern Okinawa Trough.

Deep-sea ostracod faunal dynamics in a marginal sea: biotic response to oxygen variability and mid-Pleistocene global changes

AbstractDeep-sea benthic ostracod assemblages covering the last 2 Myr were investigated in Integrated Ocean Drilling Program Site U1426 (at 903 m water depth) in the southern Sea of Japan. Results show that (1) orbital-scale faunal variability has been influenced by eustatic sea-level fluctuations and oxygen variability and (2) secular-scale faunal transitions are likely associated with the mid-Brunhes event (MBE, ~0.43 Ma) and the onset of the Tsushima Warm Current (TWC, ~1.7 Ma).Krithe,Robertsonites, andAcanthocythereisare the three most abundant genera throughout the core, accounting for 78.5% of total specimens. Multiple-regression tree analysis indicated that the TWC, the MBE, and oxygen content are the significant controlling factors of ostracod dominance. Changes in assemblages exhibit decline and recovery patterns corresponding to orbital-scale cyclicity of sea-level changes. In the Sea of Japan marginal ocean setting, this cyclicity shows a close relationship with bottom-water oxygen variability since the onset of the TWC influx. The MBE amplified the influence of the TWC and oxygen variability to the deep-sea ecosystem through larger sea-level fluctuations.Acanthocythereis dunelmensis, a circumpolar species, dominates before the TWC onset. After the TWC onset and during the mid-Pleistocene transition (MPT, ~1.2–0.7 Ma)Krithespp., known for their low-oxygen tolerance, substantially increase under moderate oxygen depletion. At the end of the MPT,Krithedominance diminishes and is replaced byRobertsonites hanaiiandPropontocyprisspp. after the MBE. The post-MBE assemblage, characterized byR. hanaii, suggests a slightly warmer environment under the development of the TWC. In addition, the post-MBE high-amplitude climate system may have caused the increased abundance of active-swimmingPropontocyprisspp. due to their superior migration ability. Benthic ecosystems in marginal seas are sensitive and vulnerable to both short- and long-term climatic changes, and the MBE is suggested to be a global biotic event affecting benthic ecosystems substantially.

North Atlantic Midlatitude Surface‐Circulation Changes Through the Plio‐Pleistocene Intensification of Northern Hemisphere Glaciation

AbstractThe North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the Last Glacial Maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of Northern Hemisphere glaciation (iNHG) at ~3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program Site U1313 (41°N) spanning ~2.8–2.4 Ma to trace the influence of Subarctic Front waters above this mid‐latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater δ18O using paired Mg/Ca‐δ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganization in the midlatitude North Atlantic analogous to the LGM. Our findings suggest that LGM‐like processes sensu stricto cannot be invoked to explain interglacial‐glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest that nutrient injection from cold‐core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma.

A 9 million-year-long astrochronological record of the early–middle Eocene corroborated by seafloor spreading rates

The early-middle Eocene (ca. 56-41 Ma) is recorded in the pelagic Scaglia Rossa and Variegata Formations of the Umbria-Marche Basin (central Italy). Geochemical and magnetostratigraphic alignment between the Bottaccione section (Gubbio, central Italy) and the Smirra core (Cagli, central Italy) allows us to generate a continuous and wellpreserved new record that, combined with previously published data from the same area, creates a continuous high-resolution record from the Paleocene-Eocene thermal maximum (ca. 56 Ma) to the lower part of chron C21n. Comparison with carbon isotope records from Ocean Drilling Program Sites 1258 and 1263 reveals a satisfactory match, providing further evidence of the global significance of the long-term trend and superposed perturbations captured by the δ13C records. The identification of astronomically forced geochemical cycles allows us to develop a 405 k.y. tuned age model, thereby extending the astrochronology from ca. 56.0 to ca. 47.5 Ma. Marine magnetic anomaly profiles from major oceanic basins characterized by high seafloor spreading rates were used to independently test the astronomical polarity time scale associated with our tuning, as well as other polarity time scales. Our age model suggests the existence of periods of relatively constant seafloor spreading rates separated by rapid changes, while the other time scales generate more gradual variations and also include large and short-term deviations in spreading rates that occur simultaneously in different oceanic basins, implying errors in polarity reversal ages. The Umbria- Marche age model further contributes to the closure of the middle Eocene gap in the astronomical time scale.

Globoconella Pseudospinosa, N. Sp.: a New Early Pliocene Planktonic Foraminifera from the Southwest Pacific

AbstractStudies of topotypes of Truncorotalia crassaconica from the East Coast Basin of New Zealand and populations of Tr. crassaconica at Ocean Drilling Program Site 1123 and Deep Sea Drilling Project Site 284 in the southwest Pacific revealed two morphospecies. One of these is a late Pliocene species, Truncorotalia crassaconica s.s., that has low, slit-like apertures with thin, smooth apertural lips. The other is Globoconella pseudospinosa n. sp., an early Pliocene form with arched apertures and pustulose apertural lips. The short stratigraphic ranges of both species are potentially useful as biostratigraphic markers for the subdivision of Plio-Pleistocene stages in New Zealand and the temperate southwest Pacific. Populations of Gc. pseudospinosa, with mostly sinistrally coiled specimens, occur throughout the lectostratotype of the New Zealand Opoitian Stage at Mangapoike River in the East Coast Basin and are confined to the early Pliocene (Opoitian) at sites 1123 and 284, between 4.57 and 4.10 Ma. Small populations of Tr. crassaconica s.s., with mostly dextrally coiled specimens, occur in the late Pliocene (Waipipian Stage) between 3.53 and 2.98 Ma. Sporadic occurrences of relatively rare, mostly sinistrally coiled specimens of Tr. aff. crassaconica occur earlier at sites 1123 and 284 in the late Miocene to Pliocene between ca. 5.5 and 3.53 Ma. These specimens are distinguished in having open umbilici and almost straight umbilical sutures, similar to the late Miocene specimens of Truncorotalia crassaformis.

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.

Suspected meteorite fragments in marine sediments from East Antarctica

AbstractUnusual mafic rock fragments deposited in Plio-Pleistocene-aged marine sediments were recorded at Integrated Ocean Drilling Program (IODP) Site U1359, in Wilkes Land, East Antarctica. These fragments were identified from sediment layers deposited between c. 3 and 1.2 Ma, indicating a sustained supply during this time interval. Clinopyroxenes in these basalts are Al–Ti diopside–hedenbergite, uncommon in terrestrial magmatic rocks. A single strong peak in the Raman spectra of a phosphate-bearing mineral at 963 cm-1 supports the presence of merrillite. Although not conclusive, petrological traits and oxygen isotopic compositions also suggest that the fragments may be extra-terrestrial fragments affected by shock metamorphism. Nevertheless, it is concluded that the basaltic fragments incorporated in marine sediments at Site U1359 represent ice-rafted material supplied to the continental rise of East Antarctica, probably from the bedrocks near the proximal Ninnis Glacier. Further studies on Plio-Pleistocene sediments near Site U1359 are required to characterize the unusual mafic rocks described.

Rock magnetism of quartz and feldspars chemically separated from pelagic red clay: a new approach to provenance study

Magnetic mineral inclusions in silicates are widespread in sediments as well as in igneous rocks. Because they are isolated from surrounding environment, they have potential to preserve original magnetic signature even in chemically altered sediments. Such inclusions may provide proxies to help differentiating the source of the host silicate. We measure magnetism of quartz and feldspars separated by chemical digestion of pelagic red clay. The samples are from the upper 15 m of sediments recovered at Integrated Ocean Drilling Program Site U1366 in the South Pacific Gyre. The quartz and feldspars account for 2.3–22.7 wt% of the samples. X-ray diffraction analyses detect both plagioclase feldspar and potassium feldspar. Plagioclase is albite-rich and abundant in the top ~ 7.4 m of the core. Potassium feldspar mainly occurs below ~ 10.4 m. The dominance of albite-rich plagioclase differs from a previous investigation of coarser fraction of sediments from the South Pacific. Saturation isothermal remanence (SIRM) intensities of the quartz and feldspars are 7.45 × 10−4 to 1.98 × 10−3 Am2/kg, accounting for less than 1.02% of the SIRM of the untreated bulk samples. The depth variations of the silicate mineralogy and the previously reported geochemical end-member contributions indicate that quartz and/or plagioclase above 8.26 m is likely to be Australian dust. In contrast, the relative abundance and the magnetic properties of quartz and feldspars vary below 10.42 m, without clear correlation with geochemical end-member contributions. We consider that these changes trace a subdivision of the volcanic component. Our results demonstrate that magnetism of inclusions can reveal additional information of mineral provenance, and chemical separation is an essential approach to reveal the environmental magnetic information carried by magnetic inclusions.

Increased seasonality and aridity drove the C4 plant expansion in Central Asia since the Miocene–Pliocene boundary

Continuous and high-resolution records of the content, mass accumulation rate (MAR) and δ13C values of black carbon obtained from Integrated Ocean Drilling Program (IODP) Site U1430 in the southwestern Japan Sea have been established and combined with previous results obtained from Central Asia. The main objective of this work is to reconstruct the historical changes in vegetation types (C3–C4), and to constrain the driving force of C4 plant expansion over the last 13 Ma. The stable carbon isotope value of black carbon (δ13CBC) shows a major shift since the Miocene–Pliocene boundary (∼5.3 Ma), suggesting significant expansion of C4 plants in broad areas of Central Asia, including the inland basins of northwestern China and the Loess Plateau. However, a decline in the content and MAR of black carbon reveals the absence of any link between fire and C4 plant expansion in Central Asia, due to the dramatic decrease in biomass under a drying regime. On a global scale, asynchronous expansion of C4 plants suggests that regional hydroclimatic change, rather than decline in CO2 concentration, was the most important factor to influence C4 expansion. We propose that the increased seasonality and the enhanced long-term aridity driven by the concurrent decline in winter westerly vapor, and increase in East Asian summer monsoon precipitation, were the main driving forces of C4 plant expansion in broad areas of Central Asia. Variations in winter westerly moisture have played a significant role in changes of regional climate and vegetation in Central Asia since the late Miocene.

Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 3: Insights from Oligocene–Miocene TEX<sub>86</sub>-based sea surface temperature reconstructions

Abstract. The volume of the Antarctic continental ice sheet(s) varied substantially during the Oligocene and Miocene (∼34–5 Ma) from smaller to substantially larger than today, both on million-year and on orbital timescales. However, reproduction through physical modeling of a dynamic response of the ice sheets to climate forcing remains problematic, suggesting the existence of complex feedback mechanisms between the cryosphere, ocean, and atmosphere systems. There is therefore an urgent need to improve the models for better predictions of these systems, including resulting potential future sea level change. To assess the interactions between the cryosphere, ocean, and atmosphere, knowledge of ancient sea surface conditions close to the Antarctic margin is essential. Here, we present a new TEX86-based sea surface water paleotemperature record measured on Oligocene sediments from Integrated Ocean Drilling Program (IODP) Site U1356, offshore Wilkes Land, East Antarctica. The new data are presented along with previously published Miocene temperatures from the same site. Together the data cover the interval between ∼34 and ∼11 Ma and encompasses two hiatuses. This record allows us to accurately reconstruct the magnitude of sea surface temperature (SST) variability and trends on both million-year and glacial–interglacial timescales. On average, TEX86 values indicate SSTs ranging between 10 and 21 ∘C during the Oligocene and Miocene, which is on the upper end of the few existing reconstructions from other high-latitude Southern Ocean sites. SST maxima occur around 30.5, 25, and 17 Ma. Our record suggests generally warm to temperate ocean offshore Wilkes Land. Based on lithological alternations detected in the sedimentary record, which are assigned to glacial–interglacial deposits, a SST variability of 1.5–3.1 ∘C at glacial–interglacial timescales can be established. This variability is slightly larger than that of deep-sea temperatures recorded in Mg ∕ Ca data. Our reconstructed Oligocene temperature variability has implications for Oligocene ice volume estimates based on benthic δ18O records. If the long-term and orbital-scale SST variability at Site U1356 mirrors that of the nearby region of deep-water formation, we argue that a substantial portion of the variability and trends contained in long-term δ18O records can be explained by variability in Southern high-latitude temperature and that the Antarctic ice volume may have been less dynamic than previously thought. Importantly, our temperature record suggests that Oligocene–Miocene Antarctic ice sheets were generally of smaller size compared to today.

10Be/9Be Ratios Reflect Antarctic Ice Sheet Freshwater Discharge During Pliocene Warming

AbstractAlong glaciated margins, ratios of meteoric cosmogenic beryllium‐10, 10Be, normalized to its stable isotope, 9Be, reflect an environmental signal, driven ultimately by climatic change. We explore the application of this isotopic pair as a proxy for East Antarctic Ice Sheet dynamics. We analyze 10Be/9Be in middle Pliocene glaciomarine sediments offshore the Wilkes Land Region (Integrated Ocean Drilling Program (IODP) Site U1361A) and examine our new record alongside existing biochemical/geochemical records (Ba/Al, opal %wt, εNd, and 87Sr/86Sr). 10Be/9Be ratios reach local maxima during pulsed, mild warming events and are strongly correlated with existing records that indicate concurrent ice sheet retraction and increased bioproductivity. We suggest climate change as the primary driver of the 10Be/9Be record near glaciated margins, whereby increased warming drives ice sheet retraction, discharging freshwaters and diluting the open ocean 10Be/9Be signal recorded in authigenic minerals.

Relative Paleointensity and Inclination Anomaly Over the Last 8 Myr Obtained From the Integrated Ocean Drilling Program Site U1335 Sediments in the Eastern Equatorial Pacific

AbstractFor understanding the fundamentals of the geodynamo such as the relation between paleointensity and polarity length and time‐averaged field (TAF) structure, continuous records of relative paleointensity (RPI) and inclination anomaly (ΔI) are desired; however, available records older than ~3 Ma are still very limited in time and space. We conducted a paleomagnetic study of the Integrated Ocean Drilling Program Site U1335 sediments in the eastern equatorial Pacific to obtain continuous RPI and ΔI records since ~8 Ma. Slow deposition, ~8.4 m/Myr or less, limits the resolution of the records but did allow for determination of long‐term variations. Rock‐magnetic measurements showed that biogenic magnetite dominates the magnetic mineral assemblages, and the proportion of biogenic to terrigenous magnetic minerals increases prior to ~4 Ma. The average paleointensity between ~4 and 8 Ma is approximately 30% lower than that from 0 to ~4 Ma. The apparent reduction of RPI at ~4 Ma reaches approximately ~50%, but ~20% of this is estimated to be artificial, induced by the increase in the proportion of biogenic magnetite. No relation between paleointensity and polarity length is recognized for the last ~8 Myr. The magnitude of ΔI is slightly larger during reversed polarity chrons (4.43° ± 1.47°) than normal polarity chrons (−0.69° ± 2.98°) over the last ~5 Myr, which agrees with the available TAF models of this time span. Prior to ~6 Ma, the sign of ΔI during the normal chrons might have switched to positive, and ΔI during reversed chrons might have been slightly larger than that after ~5 Ma.

A 1 Ma sea surface temperature record from the North Atlantic and its implications for the early human occupation of Britain

AbstractThe British Quaternary sequence has an exceptionally rich record of Palaeolithic archaeology up to 1 Ma. In this study, we reinvestigate foraminifera-based sea surface temperature (SST) reconstructions from the two marine core records (Ocean Drilling Program Site 980 and M23414) that are most relevant to the climatic history of the British Isles, consequently allowing the evolution of SST over the past 1 Ma to be studied. This is then compared with long-term changes with the British archaeological record in order to understand in greater detail the changing patterns of climatic forcing and the major climatic transitions that were the background environmental drivers against which patterns of early human occupation occurred. These include the mid-Pleistocene revolution, the mid-Brunhes event, and changing patterns of isotopic substage complexity. Significantly, however, the SST record indicates that the Marine Oxygen Isotope Stage 15–13 interval was characterised by the most prolonged period of consistently warm conditions of the entire 1 Ma interval in the northeast Atlantic. This unique climatic period correlates with the first major proliferation of archaeological sites in northwest Europe. The article concludes by discussing the significance of these climatic shifts for our understanding of early human occupation in this region.

Significant seasonal contrast in the Arabian Sea during deglaciation: Evidence from oxygen isotopic analyses of individual planktic foraminifera

The variation of stable isotopes between individual shells of planktic foraminifera of a given species and size may provide short-term seasonal insight into past sea surface temperature (SST) and salinity (SSS). In this context, oxygen isotope analyses of individual Globigerinoides sacculifer were carried out from the Ocean Drilling Program Site 723A in the western Arabian Sea to unravel seasonal changes over the last 22 kyr. δ18O values of single shells of G. sacculifer range from of 0.54–2.09‰ at various discrete depths in the core which spans the last 22 kyr. Maximum inter-shell δ18O variability and relatively higher standard deviation is noticed from 20 to 10 kyr (deglaciation), whereas from 10 kyr onwards (Holocene) inter-shell δ18O variability decreases. Based on statistical inferences, we interpret this result as greater seasonal SST and SSS contrasts during the deglaciation compared to the Holocene. These winter-to-summer contrasts during the deglaciation can likely be explained by a net reduction in summer upwelling linked to changes in wind forcing and net evaporation-minus-precipitation that might, along with local causes, be related to deglacial shifts in the intertropical convergence zone (ITCZ). We note that a common winter-dominated SST signal in the Arabian Sea and in Greenland was pervasive during the last glacial period, strengthening evidence of a North Atlantic link with Indian Ocean SST. Thus, the present study has wider implications in understanding whether the forcing mechanism of tropical monsoon climate lies in high latitudes or in the tropics.

Occurrence of benthic foraminifers across the Jurassic/Cretaceous transition in Gyangze, southern Xizang (Tibet), China

A relatively low diversity, bathyal benthic foraminiferal assemblage was recovered from the Lower Cretaceous Rilang and Gyabula formations at Chuangde near Gyangze, southern Xizang (Tibet). The assemblage contains 16 taxa belonging to 14 genera, including epifaunal forms dominated by detritus feeders, such as Ammodiscus tenuissimus, Glomospira charoides and Glomospirella gaultina, and rare tubular forms considered to be suspension-feeders such as Rhizammina indivisa and Nothia excelsa. The infaunal component is dominated by Textulariopsis sp. and Pseudobolivina sp., with rare Psammosphaera sp., Haplophragmoides sp. and Reophax sp., previously assigned to a “Biofacies B” indicative of a mildly oxygen-deficient deep water environment and increased surface water productivity. However, the rare occurrence of robust calcareous forms indicates that taphonomic processes considerably altered the original assemblage. The occurrence of the age diagnostic small morphotype of Lenticulina ouachensis in the upper Rilang Formation indicates a late Valanginian–early Hauterivian age, allowing correlation with coeval sections from the Vocontian Trough, northern Tethys (Bavarian Alps) and eastern Indian Ocean (Ocean Drilling Program Site 766 off northwestern Australia).

Calcareous plankton and the mid-Brunhes climate variability in the Alboran Sea (ODP Site 977)

Quantitative calcareous plankton analyses have been performed at the Ocean Drilling Program Site 977 in the Alboran Sea through the mid-Brunhes interval (300–540 ka). The results evidence orbital-suborbital and millennial scale climate variability which primary reflects temperature change. The patterns of increase/decrease of coccolithophore and planktonic foraminifera warm water taxa reproduce the stage and substage climate variability in the planktonic δ18O in good agreement with other planktonic isotope records from Mediterranean Sea and Atlantic Ocean. Variable conditions, characterized by alternating cool nutrient-rich or warm and stable sea-surface waters, occurred during interglacial Marine Isotope Stage (MIS) 13 in response to high amplitude oscillations of insolation during obliquity maximum and to North Africa monsoon strength, influencing distribution of taxa. MIS 13a resulted warmer than MIS 13c based on the higher abundances of warm water taxa; they increase just after insolation maxima, suggesting a primary response of calcareous plankton to precessional orbital control. Following short-term warm (MIS 11e) and more humid (MIS 11d) phases, a climate optimum lasting about 16 kyr is recognized during MIS 11c, when major tropical-subtropical water arrival into the Alboran Sea occurred as evidenced by the presence of Trilobatus sacculifer. MIS 11c records unstable climate condition that persisted up to the late MIS 11, indicating multiple pulses of southward migrations of subpolar front following the full interglacial MIS 11c. Warm and oligotrophic conditions occurred during MIS 9 as during MIS 11, especially in MIS 9e, although without a climate optimum. Reduced abundance of warm water taxa and decrease of Globorotalia inflata highlight colder surface water conditions and decreased Atlantic-Mediterranean water exchange during glacials, especially during the prominent sea-level low-stand of MIS 12. Short-term prominent peaks of the polar-subpolar Neogloboquadrina pachyderma and Coccolithus pelagicus ssp. pelagicus, and additional key taxa preferring fresher and turbid water, together with minima in total Nannofossil Accumulation Rate and enhanced coccolith reworking, sustain a correlation with the North Atlantic Heinrich-type (Ht) events Ht1, Ht1a, Ht3, Ht4-6, thus documenting the sensitiveness of the Alboran Sea in recording European and Laurentide ice-sheet dynamics. The comparable chronology of most of these cold spells in the available Balearic and Ionian calcareous plankton records highlights that Atlantic melt water entering Alboran basin through Gibraltar Strait may have reached the central Mediterranean preserving some of its original polar signature.