Mid Holocene origin of the sea-surface salinity low in the subarctic North Pacific

IMAGES core MD01-2416 (51°N, 168°E) provides the first centennial-scale multiproxy record of Holocene variation in North Pacific sea-surface temperature (SST), salinity, and biogenic productivity. Our results reveal a gradual decrease in subarctic SST by 3–5 °C from 11.1 to 4.2 ka and a stepwise long-term decrease in sea surface salinity (SSS) by 2–3 p.s.u. Early Holocene SSS were as high as in the modern subtropical Pacific. The steep halocline and stratification that is characteristic of the present-day subarctic North Pacific surface ocean is a fairly recent feature, developed as a product of mid-Holocene environmental change. High SSS matched a salient productivity maximum of biogenic opal during Bølling-to-Early Holocene times, reaching levels similar to those observed during preglacial times in the warm mid-Pliocene prior to 2.73 Ma. Similar productivity spikes marked every preceding glacial termination of the last 800 ka, indicating recurrent short-term events of mid-Pliocene-style intense upwelling of nutrient-rich Pacific Deepwater in the Pleistocene. Such events led to a repeated exposure of CO₂-rich deepwater at the ocean surface facilitating a transient CO₂ release to the atmosphere, but the timing and duration of these events repudiate a long-term influence of the subarctic North Pacific on global atmospheric CO₂ concentration.     

A late Pleistocene human presence at Huaca Prieta,Peru, and early Pacific Coastal adaptations

Archaeological excavations in deep pre-mound levels at Huaca Prieta in northern Peru have yielded new evidence of late Pleistocene cultural deposits that shed insights into the early human occupation of the Pacific coast of South America. Radiocarbon dates place this occupation between ~ 14,200 and 13,300 cal yr BP. The cultural evidence shares certain basic technological and subsistence traits, including maritime resources and simple flake tools, with previously discovered late Pleistocene sites along the Pacific coast of Peru and Chile. The results help to expand our knowledge of early maritime societies and human adaption to changing coastal environments.     

Persistent sea surface temperature and declined sea surface salinity in the northwestern tropical Pacific over the past 7500 years

To understand Holocene climate evolutions in low-latitude region of the western Pacific, paired δ¹⁸O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber (250–300 μm, sensu stricto, s.s.) from a marine core ORI715-21 (121.5°E, 22.7°N, water depth 760 m) underneath the Kuroshio Current (KC) off eastern Taiwan were analyzed. Over the past 7500 years, the geochemical proxy-inferred sea surface temperature (SST) hovered around 27–28 °C and seawater δ¹⁸O (δ¹⁸O_W) slowly decreased 0.2–0.4‰ for two KC sites at 22.7° and 25.3°N. Comparison with a published high-SST and high-salinity equatorial tropical Pacific record, MD98-2181 located at the Mindanao Current (MC) at 6.3°N, reveals an anomalous time interval at 3.5–1.5 kyr ago (before 1950 AD). SST gradient between the MC site and two KC site decrease from 1.5–2.0 °C to only 0–1 °C, and δ¹⁸O_W from 0.1–0.3‰ to 0‰ for this 2-kyr time window. The high SST and low gradient could result from a northward shift of the North Equatorial Current, which implies a weakened KC. The long-term descending δ¹⁸O_W and increasing precipitation in the entire low-latitude western Pacific and the gradually decreasing East Asian summer monsoonal rainfall during middle-to-late Holocene is likely caused by different land and ocean responses to solar insolation and/or enhanced moisture transportation from the Atlantic to Pacific associated with the southward movement of ITCZ.     

Evidence for prolonged El Nino-like conditions in the Pacific during the Late Pleistocene: a 43 ka noble gas record from California groundwaters

Information on the ocean/atmosphere state over the period spanning the Last Glacial Maximum – from the Late Pleistocene to the Holocene – provides crucial constraints on the relationship between orbital forcing and global climate change. The Pacific Ocean is particularly important in this respect because of its dominant role in exporting heat and moisture from the tropics to higher latitudes. Through targeting groundwaters in the Mojave Desert, California, we show that noble gas derived temperatures in California averaged 4.2 ± 1.1 °C cooler in the Late Pleistocene (from ～43 to ～12 ka) compared to the Holocene (from ～10 to ～5 ka). Furthermore, the older groundwaters contain higher concentrations of excess air (entrained air bubbles) and have elevated oxygen-18/oxygen-16 ratios (δ¹⁸O) – indicators of vigorous aquifer recharge, and greater rainfall amounts and/or more intense precipitation events, respectively. Together, these paleoclimate indicators reveal that cooler and wetter conditions prevailed in the Mojave Desert from ～43 to ～12 ka. We suggest that during the Late Pleistocene, the Pacific ocean/atmosphere state was similar to present-day El Nino-like patterns, and was characterized by prolonged periods of weak trade winds, weak upwelling along the eastern Pacific margin, and increased precipitation in the southwestern U.S.     

Climate forcing,primary production and the distribution of Holocene biogenic sediments in the Gulf of California

The Gulf of California is a marginal seaway under the influence of a monsoon climate that produces cool, dry winters and warm, humid summers. Winds, tidal mixing and coastal-trapped waves forced by climate and the Pacific Ocean control nutrient advection and primary productivity (PP). Strong northwest winds from the subtropical East Pacific High Pressure system begin in November and last until April and drive coastal upwelling along the mainland margin, especially in the central and southern Gulf. In the northern Gulf, particularly around the midrift island, tidal mixing and turbulence occurs year round, advecting nutrients into the mixed layer and high productivity. During summer and early fall months, winds are variable, of less intensity and mainly blow cross-basin except in the most northern Gulf. Summer PP is generally low in the central and southern Gulf except along the mainland where coastal-trapped waves associated with tropical surges and hurricanes generate mixing over the continental shelf. Mesoscale eddies or gyres often associated with jets and filaments extend to depths of 1000 m and transport nutrient-enriched upwelled waters and plankton detritus across the Gulf. The largest and most persistent gyres rotate in an anti-cyclonic direction (east to west) and are a principal source of the plankton export to the peninsula margin.Two major biogenic sediment patterns are present in core-top sediments. Hemipelagic biosiliceous-rich muds are accumulating beneath upwelling areas of high productivity in the central Gulf and along the mainland margin. Calcium carbonate- and organic carbon-rich (OC) sediments are concentrated along the peninsula margin, generally beneath lower productivity waters with the highest OC content in areas with the lowest productivity. The high, uniform biosiliceous content in Guaymas basin, extending southward into Carmen basin reflects the redistribution by mesoscale gyres of phytoplankon debris produced in mainland coastal upwelling and tidally forced areas around the midrift islands.Holocene biogenic patterns are similar to the present day with the major difference in rates of accumulation. Phytoplankton production prior to about 8200 yr BP was significantly higher in the central and southern Gulf, decreased though the mid-Holocene and has been reasonably steady for the past 2500 yr. The strong north–south and east–west gradients in present-day phytoplankton productivity patterns are also reflected in the Holocene sediment record. A series of depositional cycles occur in the biogenic record with the strongest peaks of variability at about 150 (144±18), 200 (198±5) and 350 (350±40) yrs. Longer periodicities are present prior to 3200 yr BP but the 350 yr cycle dominates in the late Holocene where it is best expressed as productivity/dissolution cycles in the carbonate record.     

Tectonic evaluation of the Indochina Block during Jurassic-Cretaceous from palaeomagnetic results of Mesozoic redbeds in central and southern Lao PDR

Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A₉₅ = 2.6°), 63. 3°N/170.2°E (A₉₅ = 6.9°), and 67.0°N/180.8°E (A₉₅ = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.     

Tropical northeast Africa in the middle–late Eocene: Paleomagnetism of the marine-mammals sites and basalts in the Fayum province,Egypt

The mid-late Eocene “Valley of Whales” in the Fayum province of Egypt contains hundreds of marine-mammals’ skeletons. Given its paleontological importance, we carried out a paleomagnetic study of the fossil-bearing formations. A sequence of basalts directly overlying the upper Eocene rocks in three distant clusters within a 25 km-long NW–SE graben in the southwestern part of the area was also studied. Thermal demagnetization of three-axis IRM was used to identify and eliminate sites dominated by hematite and/or goethite as potential remanence carriers. Progressive thermal demagnetization of the NRM isolated a characteristic NNE–SSW dual-polarity direction with a shallow inclination that passes both tilt and reversal tests. The mean tilt-corrected direction of the sedimentary formations is D/I = 16°/30° (k = 50, α₉₅ = 3°) yielding a paleomagnetic pole at 70°N/159°E. The anisotropy of magnetic susceptibility (AMS) indicated that the observed inclinations were free from inclination shallowing, as did the nearly identical characteristic remanence of the overlying basalt flows (with a tilt-corrected reversed-polarity direction of D/I = 198°/−28° (k = 38, α₉₅ = 7°) and a pole at 68°N/158°E). The new paleopoles place the Fayum province at a lower paleolatitude (15–17°N) than today (29.5°N), and point to the possible prevalence of tropical climate in northeast Africa during mid-late Eocene times. This tropical position is nearly identical to the paleolatitudes extrapolated from the mean of 36 coeval poles rotated from the other major cratons and from Africa itself. The declinations show a minor easterly deviation from those predicted by extrapolation from other continents. This is interpreted as due to a small clockwise rotation internal to NE Africa, possibly related to Red Sea/Gulf of Suez rifting after the late Eocene. The alternative explanation that the geomagnetic field had a non-zonal non-dipole field contribution is not favored.     

The optimal windows for seismically-enhanced gold precipitation in the epithermal environment

Epithermal gold (Au) deposits result from the combination of a sustained flux of metal-rich fluids and an efficient precipitation mechanism. Earthquakes may trigger gold precipitation by rapid loss of fluid pressure but their efficiency and time-integrated contribution to gold endowment are poorly constrained. In order to quantify the feedbacks between earthquake-driven fracturing and gold precipitation in the shallow crust, we studied the gold-rich fluids in the active Tolhuaca geothermal system, located in the highly seismic Southern Andes of Chile. We combined temperature measurements in the deep wells with fluid inclusion data, geochemical analyses of borehole fluids and numerical simulations of coupled heat and fluid flow to reconstruct the physical and chemical evolution of the hydrothermal reservoir. The effect of seismic perturbations on fluid parameters was constrained using a thermo-mechanical piston model that simulates the suction pump mechanism occurring in dilational jogs. Furthermore, we evaluated the impact of fluid parameters on gold precipitation by calculating the solubility of gold in pressure (P)–enthalpy (H) space. The reconstructed fluid conditions at Tolhuaca indicate that single-phase convective fluids feeding the hydrothermal reservoir reach the two-phase boundary with a high gold budget (~ 1–5 ppb) at saturated liquid pressures between 20 and 100 bar (210 °C < T_sat < 310 °C). We show that if hydrothermal fluids reach this optimal threshold for gold precipitation at a temperature near 250 °C, small adiabatic pressure drops (~ 10 bar) triggered by transient fault-rupture can produce precipitation of 95% of the dissolved gold. Our results at the active Tolhuaca geothermal system indicate that subtle, externally-forced perturbations – equivalent to low magnitude earthquakes (Mw < 2) of a hydrothermal reservoir under optimal conditions – may significantly enhance gold precipitation rates in the shallow crust and lead to overall increases in metal endowment over time.     

Tectonics and thermal structure of western Satpura,India

Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity (P_n) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m^?2 has been estimated based on P_n velocity, which agrees with the value of heat flow of 52 ± 4 mW m^?2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.     

Paleomagnetic analysis of the Marwar Supergroup,Rajasthan, India and proposed interbasinal correlations

The Marwar Supergroup refers to a 1000–2000 m thick marine and coastal sequence that covers a vast area of Rajasthan in NW–India. The Marwar Basin uncomformably overlies the ∼750–770 Ma rocks of the Malani Igneous Suite and is therefore considered Late Neoproterozoic to Early Cambrian in age. Upper Vindhyan basinal sediments (Bhander and Rewa Groups), exposed in the east and separated by the Aravalli–Delhi Fold Belt, have long been assumed to coeval with the Marwar Supergroup. Recent studies based on detrital zircon populations of the Marwar and Upper Vindhyan sequences show some similarity in the older populations, but the Vindhyan sequence shows no zircons younger than 1000 Ma whereas samples taken from the Marwar Basin show distinctly younger zircons. This observation led to speculation that the Upper Vindhyan and Marwar sequences did not develop coevally.While there are alternative explanations for why the two basins may differ in their detrital zircon populations, paleomagnetic studies may provide independent evidence for differences/similarities between the assumed coeval basins. We have collected samples in the Marwar Basin and present the paleomagnetic results. Previous paleomagnetic studies of Marwar basinal sediments were misinterpreted as being indistinguishable from the Upper Vindhyan sequence. The vast majority of our samples show directional characteristics similar to the previously published studies. We interpret these results to be a recent overprint. A small subset of hematite-bearing rocks from the Jodhpur Formation (basal Marwar) exhibit directional data (Dec = 89° Inc = −1° α95 = 9°) that are distinct from the Upper Vindhyan pole and may offer additional support for temporally distinct episodes of sedimentation in these proximal regions. A VGP based upon our directional data is reported at 1°S 344°E (dp = 5°, dm = 9°). We conclude that the Marwar Supergroup developed near the close of the Ediacaran Period and is part of a larger group of sedimentary basins that include the Huqf Supergroup (Oman), the Salt-Range (Pakistan), the Krol–Tal belt (Himalayas) and perhaps the Molo Supergroup (Madagascar).     

Climate and vegetation in Hokkaido, northern Japan, since the LGM: Pollen records from core GH02-1030 off Tokachi in the northwestern Pacific

Vegetation and climate since the LGM in eastern Hokkaido were investigated based on a pollen record from marine core GH02-1030 from off Tokachi in the northwestern Pacific. We also examined pollen spectra in surface samples from Sakhalin to compare and understand the climatic conditions of Hokkaido during the last glacial period. Vegetation in the Tokachi region in the LGM (22–17 ka) was an open boreal forest dominated by Picea and Larix. During the last deglaciation (17–10 ka), vegetation was characterized by abundant Betula. In the Kenbuchi Basin, central Hokkaido, a remarkable increase of Larix and Pinus occurred in the LGM and the last deglaciation, which was assigned as the “Kenbuchi Stadial.” Comparison of climatic data between the core GH02-1030 and that of Kenbuchi Basin demonstrates that variations in temperature and precipitation were larger in inland Hokkaido than in the maritime area of the Pacific coast. During the LGM in the Tokachi region, the August mean temperature was about 5 °C lower, and annual precipitation was about 40% lower than today. In the Kenbuchi Basin, central Hokkaido, the August mean temperature was about 8 °C lower, and annual precipitation was half that of today. During the last deglaciation, August mean temperatures were about 3 °C lower, and annual precipitation was about 30% lower than today in the Tokachi region. In the Kenbuchi Basin, August mean temperatures were about 5–8 °C lower, and annual precipitation was about 40–60% lower than today. Cold ocean water and a strengthened summer monsoon after 15 ka may have resulted in the formation of advection fogs, reduced summer temperatures, and a decrease in the seasonal temperature difference in the Tokachi district, which established favorable maritime conditions for Betula forests.     

Kinematics of the crust around the Tanggula Shan in North–Central Tibet: Constraints from paleomagnetic data

We have conducted a paleomagnetic investigation on the Middle–Upper Jurassic marine strata exposed in the hanging wall of the Tanggula Thrust system near the Yanshiping area, northern Tibet. Progressive demagnetization experiments successfully isolated stable magnetization over a broad spectrum of demagnetization temperatures. The mean direction of the characteristic remanent magnetizations for the Middle–Late Jurassic Yanshiping Group in stratigraphic coordinates (D/I (Declination/Inclination) = 5.6°/60.3°, k = 22.9, α95 = 12.9°, N = 7 s) is much more clustered than the mean direction in geographic coordinates (D/I = 345.5°/37.2°, k = 2.5, α95 = 48.4°), indicating magnetization was not acquired after folding. Although the conventional fold test is positive, incremental untilting test on the characteristic remanent magnetization reveals that a maximum value of precision parameter k occurs at 82.1 ± 4.6% untilting (D/I = 3.3°/57.8°, k = 43.9, α95 = 9.2°), which indicates the ChRMs are probably acquired during Late Cretaceous folding. This synfolding magnetization component is therefore secondary. The corresponding pole position (84.4°N, 119.4°E with dp/dm = 13.5/9.9°) is inconsistent with Jurassic–Early Cretaceous paleopoles of the region, but the paleolatitude is consistent with the Late Cretaceous paleolatitude observed in the Qiangtang terrane and its periphery. The synfolding component is carried by both magnetite and hematite, which were identified by isothermal remnant magnetization acquisition experiments, unblocking temperatures of stable magnetic components, and Curie temperature determination and correlated with observed hydrothermal veins. Available geological evidences indicate that the synfolding magnetization is probably the result of chemical remagnetization caused by orogenic fluids or hydrothermal sources during the early uplift of the Tibetan Plateau.     

Changing marine productivity off northern Chile during the past 19 000 years: a multivariable approach

A multivariable approach utilising bulk sediment, planktonic Foraminifera and siliceous phytoplankton has been used to reconstruct rapid variations in palaeoproductivity in the Peru–Chile Current System off northern Chile for the past 19 000 cal. yr. During the early deglaciation (19 000–16 000 cal. yr BP), our data point to strongest upwelling intensity and highest productivity of the past 19 000 cal. yr. The late deglaciation (16 000–13 000 cal. yr BP) is characterised by a major change in the oceanographic setting, warmer water masses and weaker upwelling at the study site. Lowest productivity and weakest upwelling intensity are observed from the early to the middle Holocene (13 000–4000 cal. yr BP), and the beginning of the late Holocene (<4000 cal. yr BP) is marked by increasing productivity, mainly driven by silicate‐producing organisms. Changes in the productivity and upwelling intensity in our record may have resulted from a large‐scale compression and/or displacement of the South Pacific subtropical gyre during more productive periods, in line with a northward extension of the Antarctic Circumpolar Current and increased advection of Antarctic water masses with the Peru–Chile Current. The corresponding increase in hemispheric thermal gradient and wind stress induced stronger upwelling. During the periods of lower productivity, this scenario probably reversed. Copyright © 2004 John Wiley & Sons, Ltd.     

Mantle plume or slab window?: Physical and geochemical constraints on the origin of the Caribbean oceanic plateau

The Caribbean oceanic plateau formed in the Pacific realm when it erupted onto the Farallon plate from the Galapagos hotspot at ～ 90 Ma. The plateau was subsequently transported to the northeast and collided with the Great Arc of the Caribbean thus initiating subduction polarity reversal and the consequent tectonic emplacement of the Caribbean plate between the North and South American continents. The plateau represents a large outpouring of mafic volcanism, which has been interpreted as having formed by melting of a hot mantle plume. Conversely, some have suggested that a slab window could be involved in forming the plateau. However, the source regions of oceanic plateaus are distinct from N-MORB (the likely source composition for slab window mafic rocks). Furthermore, melt modelling using primitive (high MgO) Caribbean oceanic plateau lavas from Curaçao, shows that the primary magmas of the plateau contained ～ 20 wt.% MgO and were derived from 30 to 32% partial melting of a fertile peridotite source region which had a potential temperature (T_p) of 1564–1614 °C. Thus, the Caribbean oceanic plateau lavas are derived from decompression melting of a hot upwelling mantle plume with excess heat relative to ambient upper mantle. Extensional decompression partial melting of sub-slab asthenosphere in a slab window with an ambient mantle T_p cannot produce enough melt to form a plateau. The formation of the Caribbean oceanic plateau by melting of ambient upper mantle in a slab window setting, is therefore, highly improbable.     

Characterisation of Triassic rifting in Peru and implications for the early disassembly of western Pangaea

Triassic igneous and sedimentary rocks exposed within the basement of the Andes were deposited in a series of rifts, and may record the early disassembly of western Pangaea. These rocks are sporadically exposed along almost the entire length of western South America, although reliable geochronological and isotopic data are sparse. We combine new geochronological (zircon U–Pb), isotopic (Hf, Nd) and geochemical data with stratigraphic observations to constrain the age and tectonic setting of the Mitu Rift of southern Peru. The Peruvian Mitu Rift is compared with other Triassic rifts in Colombia and Ecuador (Palanda Rift; 240–225 Ma), Bolivia (Mitu Rift; Triassic), Bolivia, Chile and Argentina (e.g. Cuyo Basin; 246–230 Ma), and conclusions are reached regarding the relationship between Triassic extension along the western margin of Pangaea, and the eventual formation of the Proto-Caribbean and Central Atlantic oceans. The Mitu Rift (Peru) was active during ~ 245–240 to ~ 220 Ma and was synchronous with rifting along the Pacific margin of Colombia and Ecuador, along the Chilean margin and western Argentina, and probably rifting within Bolivia. Rifting north of the Huancabamba Deflection was accompanied by subduction and led to seafloor spreading, whereas rifting along the Peruvian and Chilean margins mainly occurred in the absence of subduction and terminated prior to the formation of extensive transitional crust. Extension within Peru and Chile probably occurred via a combination of transtension, steepening and detachment of an arrested slab. We propose that plate tectonic forces initiated the early break-up of Pangaea by attenuating its margins and enhancing mantle upwelling. Prolonged extension may have propagated along pre-existing weak zones that extended into the continental interior, captured melts derived from the upwelled mantle forming a LIP (e.g. Central Atlantic Magmatic Province), became hot and weak and eventually lead to the formation of a juvenile ocean (e.g. Central Atlantic).     

Oceanographic variations over the last 150,000 yr in the Japan Sea and synchronous Holocene with the Northern Hemisphere

The paleo-hydrography in the Japan Sea called a “mini-Ocean” was reconstructed based on the high-resolution analysis of diatom assemblages over the period of 150,000 yr. The decrease of diatom fertilization in the Japan Sea, when it was isolated from surrounding seas due to the drop of sea-level during the glacial to stadial phase, resulted in dissolution and/or extremely low diatom production in the northern cores in the subarctic water-masses. The annual Td′-derived paleo-SSTs (°C) were controlled by the fluctuations of 2-kyr and 4-kyr periods at intervals of 20 kyr and 40 kyr over the last 160 kyr BP, respectively. A 23-kyr cycle is recognized during the periods from 140 ka to 100 ka, according to the Wavelet analysis. After temperature and sea-level increased both at 133–128 ka, 60–53 ka and 15–10 ka, oceanic warm-water diatom species predominated at 127–119 ka and after 9 cal ka in the interstadial phase. At 21.3–16.9 ka and 12.9–9.8 ka, sea-level and salinity increased as the transgression developed. At 10.0–7.0 ka, the oceanic association shifted from cold-water species in the stadial period to warm-water ones in the interstadial phase. The fluctuations of Td′ derived-SSTs (°C) on century to millennial time-scales during the Holocene are well correlated with abrupt climatic events that different paleoclimatic proxies record in many regions of the Northern Hemisphere.     

Combined U-Pb,Lu-Hf,Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane,Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. ⁴⁰Ar/³⁹Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.     

Diagenetic state and source characterization of marine sediments from the inner continental shelf of the Gulf of Cádiz (SW Spain), constrained by terrigenous biomarkers

Surface sediments from the Gulf of Cádiz (GoC) were analyzed by alkaline CuO oxidation, in order to estimate the contribution of terrigenous organic matter (TOM) to the inner continental shelf of the southwest Iberian Peninsula. The parallel analysis of sediment samples from the two most important rivers draining to this coastal area (i.e. Guadiana River and Tinto–Odiel fluvial system) provided fundamental information regarding local terrestrial sources. Relatively constant intensive lignin parameters (S:V = 1.0 ± 0.1 and C:V = 0.22 ± 0.04) and high values of the lignin phenol vegetation index (LPVI = 155 ± 43) indicated that non-woody angiosperm tissues constitute the dominant component of vascular plant material reaching the shelf sediments. The NW to SE decreasing isotopic (¹³C) and molecular (Λ₈) signatures found among the sediments, coinciding with the Guadiana delivery plume, suggest that this river is the main terrestrial source in the inner GoC shelf. Slightly elevated values of degradation indicative ratios ([Ad:Al]_V = 0.41 ± 0.10; [Ad:Al])_S = 0.34 ± 0.07; [3,5-Bd:V] = 0.14 ± 0.05; P:[V + S] = 0.24 ± 0.09) suggested the alteration state of the shelf sediments. The two fold higher ratios of the river sediments (Guadiana: [Ad:Al]_V = 0.82 ± 0.08; [Ad:Al]_S = 0.84 ± 0.03; Tinto–Odiel: [Ad:Al]_V = 0.86 ± 0.12; [Ad:Al]_S = 0.83 ± 0.013) and the increasing degradation trend observed outward in the shelf, lead us to consider preferential sorption processes, instead of in situ diagenesis, to affect the degradation signature of the shelf sediments. Preferentially solubilized degraded OM is more likely to be sorbed and stabilized prior to transport to the marine system, showing an apparently more advanced degradation state. The use of the 3,5-Bd:V ratio in conjunction with (Ad:Al)_V revealed a composition continuum of the sedimentary OM ranging from fresh plant materials to highly altered soil humic constituents. Elemental and molecular analyses show a land to sea gradient by a NW to SE decrease of the terrestrial influence, accounting for larger terrestrial inputs (TOM: 71–98%) in those sediments near the Guadiana mouth, and predominantly autochthonous composition (TOM: 42–50%) in those located offshore. This work utilizes lignin derived biomarkers to determine the contribution of terrigenous OM delivered to this poorly described coastal area from regional rivers. Within a context of increasing international efforts to better understand the global C cycling, this study illustrates the relevance of using the alkaline CuO oxidation approach to evaluate C budgets and continental influence in river dominated ocean margins.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Synchronous climatic change inferred from diatom records in four western Montana lakes in the U.S. Rocky Mountains

Late-Holocene environmental and climatic conditions were reconstructed from diatom assemblages in sediment cores from four western Montana lakes: Crevice Lake, Foy Lake, Morrison Lake, and Reservoir Lake. The lakes show synchroneity in timing of shifts in diatom community structure, but the nature of these changes differs among the lakes. Two of the sites provide highly resolved records of hydrologic balance, while the other two stratigraphic sequences primarily record temperature impact on lake thermal structure. All four lakes show significant change in five discrete intervals: 2200–2100, 1700–1600, 1350–1200, 800–600, and 250 cal yr BP. The similarities in the timing of change suggest overlying regional climatic influences on lake dynamics. The 800–600 cal yr BP shift is evident in other paleorecords throughout the Great Plains and western US, associated with the transition from the Medieval Climate Anomaly to the Little Ice Age. Large-scale climatic mechanisms that influence these lake environments may result from atmospheric circulation patterns that are driven by interactions between Pacific and Atlantic sea-surface temperatures, which are then locally modified by topography.