Environmental Analysis of Cores from the Helike Delta, Gulf of Corinth, Greece

The fan delta southeast of Aigion on the southwest shore of the Gulf of Corinth was the site of ancient Helike, a city destroyed and submerged by an earthquake and seismic sea wave in 373 BC. Bore holes drilled on the Helike Delta yielded numerous ceramic fragments in the upper 12 meters, and a record of changing local environments on the delta during the Holocene period. At about 8 m below present sea level the core profiles show a general upward transition from marine to lacustrine/lagoonalc onditions.T he transition dates from about 8 kyr BP and is probably due to the deceleration of global sea level rise at the end of the last Ice Age. The deceleration apparently induced an upward and seaward progression of a zone of green clay and silt associated with brackish fauna

South China Sea Rifted Margin Testing hypotheses for lithosphere thinning during continental breakup: Drilling at the South China Sea rifted margin

International Ocean Discovery Program Expedition 368 is the second of two consecutive cruises that form the South China Sea Rifted Margin program. Expeditions 367 and 368 share the common key objectives of testing scientific hypotheses of breakup of the northern South China Sea (SCS) margin and comparing its rifting style and history to other nonvolcanic or magma-poor rifted margins. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C. They are located within the continent-ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include 1. Determining the nature of the basement within crustal units across the COT of the SCS that are critical to constrain style of rifting, 2. Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, 3. Constraining vertical crustal movements during breakup, and 4. Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively. However, based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In total, the expedition completed operations at four sites (U1501, U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing to 990 m because of mechanical problems with the drilling equipment that limited the expedition from 25 May 2017 to the end of the expedition to operate with a drill string not longer than 3400 m. New alternate Site U1504 proposed during Expedition 367 met this condition. Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already drilled Site U1501. At Site U1501, we cored to 697.1 m in 9.4 days, with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m, set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m, setting 991.5 m of 10¾ inch casing, but no cores were taken. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Site U1505, we drilled to acoustic basement, which prior to the expedition, except for Site U1501, had been interpreted to be crystalline basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment. At Site U1501 on the OMH, coring ~45 m into the acoustic basement sampled highly lithified sandstone to conglomerate of presumed Mesozoic age overlain by siliciclastic Eocene pre- to synrift sediments of Oligocene age and topped by primarily carbonaceous postrift sediments of early Miocene to Pleistocene age. Site U1502 on Ridge A was cased to 723.7 m. At this site, we recovered 180 m of hydrothermally altered brecciated basalts comprising sheet and pillow lavas below deep-marine sediments of Oligocene to late Miocene age. Coring was not performed within the upper 380 m (~Pliocene-Pleistocene) at Site U1502. At Site U1503 on Ridge C, 991.5 m of casing was installed in preparation for the planned deep drilling to ~1800 m, but no coring was performed due to mechanical failures, and the site was abandoned without further activity. Coring at Site U1504 on the OMH ~45 km east of Site U1501 recovered metamorphic schist to gneiss (greenschist facies) below late Eocene (?) carbonate rocks (partly reef debris) and early Miocene to Pleistocene sediments. At Site U1505, we cored to 480.15 m through Pleistocene to late Oligocene mainly carbonaceous ooze followed at depth by early Oligocene to late Eocene siliciclastic sediments. Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted within the Eocene to Plio-Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results. As a result of the constraints on the length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than planned because these sites are located in shallower water depths and required less penetration depth. This forced change in emphasis, however, was without fatal consequences for the primary tectonic objectives. The two expeditions together provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin

Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO2 and at the onset of the PETM

Shallow-water hydrothermal venting linked to the Palaeocene–Eocene Thermal Maximum

The Palaeocene–Eocene Thermal Maximum (PETM) was a global warming event of 5–6 °C around 56 million years ago caused by input of carbon into the ocean and atmosphere. Hydrothermal venting of greenhouse gases produced in contact aureoles surrounding magmatic intrusions in the North Atlantic Igneous Province have been proposed to play a key role in the PETM carbon-cycle perturbation, but the precise timing, magnitude and climatic impact of such venting remains uncertain. Here we present seismic data and the results of a five-borehole transect sampling the crater of a hydrothermal vent complex in the Northeast Atlantic. Stable carbon isotope stratigraphy and dinoflagellate cyst biostratigraphy reveal a negative carbon isotope excursion coincident with the appearance of the index taxon Apectodinium augustum in the vent crater, firmly tying the infill to the PETM. The shape of the crater and stratified sediments suggests large-scale explosive gas release during the initial phase of vent formation followed by rapid, but largely undisturbed, diatomite-rich infill. Moreover, we show that these vents erupted in very shallow water across the North Atlantic Igneous Province, such that volatile emissions would have entered the atmosphere almost directly without oxidation to CO2 and at the onset of the PETM

A reference time scale for Site U1385 (Shackleton Site) on the SW Iberian Margin

Variations in sediment color contain very strong precession signals at Site U1385, and the amplitude modulation of these cycles provides a powerful tool for developing an orbitally-tuned age model. We tuned the U1385 record by correlating peaks in L* to the local summer insolation maxima at 37°N. The benthic δ18O record of Site U1385, when placed on the tuned age model, generally agrees with other time scales within their respective chronologic uncertainties. The age model is transferred to down-core data to produce a continuous time series of log(Ca/Ti) that reflect relative changes of biogenic carbonate and detrital sediment. Biogenic carbonate increases during interglacial and interstadial climate states and decreases during glacial and stadial periods. Much of the variance in the log(Ca/Ti) is explained by a linear combination of orbital frequencies (precession, tilt and eccentricity), whereas the residual signal reflects suborbital climate variability. The strong correlation between suborbital log(Ca/Ti) variability and Greenland temperature over the last glacial cycle at Site U1385 suggests that this signal can be used as a proxy for millennial-scale climate variability over the past 1.5 Ma. Millennial climate variability, as expressed by log(Ca/Ti) at Site U1385, was a persistent feature of glacial climates over the past 1.5 Ma, including glacial periods of the early Pleistocene (‘41-kyr world’) when boundary conditions differed significantly from those of the late Pleistocene (‘100-kyr world’). Suborbital variability was suppressed during interglacial stages and enhanced during glacial periods, especially when benthic δ18O surpassed ~ 3.3–3.5‰. Each glacial inception was marked by appearance of strong millennial variability and each deglaciation was preceded by a terminal stadial event. Suborbital variability may be a symptomatic feature of glacial climate or, alternatively, may play a more active role in the inception and/or termination of glacial cycles

A two million year record of low-latitude aridity linked to continental weathering from the Maldives

Indian-Asian monsoon has oscillated between warm/wet interglacial periods and cool/dry glacial periods with periodicities closely linked to variations in Earth’s orbital parameters. However, processes that control wet versus dry, i.e. aridity cyclical periods on the orbital time-scale in the low latitudes of the Indian-Asian continent remain poorly understood because records over millions of years are scarce. The sedimentary record from International Ocean Discovery Program (IODP) Expedition 359 provides a well-preserved, high-resolution, continuous archive of lithogenic input from the Maldives reflecting on low-latitude aridity cycles. Variability within the lithogenic component of sedimentary deposits of the Maldives results from changes in monsoon-controlled sedimentary sources. Here, we present X-ray fluorescence (XRF) core-scanning results from IODP Site U1467 for the past two million years, allowing full investigation of orbital periodicities. We specifically use the Fe/K as a terrestrial climate proxy reflecting on wet versus dry conditions in the source areas of the Indian-Asian landmass, or from further afield. The Fe/K record shows orbitally forced cycles reflecting on changes in the relative importance of aeolian (stronger winter monsoon) during glacial periods versus fluvial supply (stronger summer monsoon) during interglacial periods. For our chronology, we tuned the Fe/K cycles to precessional insolation changes, linking Fe/K maxima/minima to insolation minima/maxima with zero phase lag. Wavelet and spectral analyses of the Fe/K record show increased dominance of the 100 kyr cycles after the Mid Pleistocene Transition (MPT) at 1.25 Ma in tandem with the global ice volume benthic δ 18 O data (LR04 record). In contrast to the LR04 record, the Fe/K profile resolves 100-kyr-like cycles around the 130 kyr frequency band in the interval from 1.25 to 2 million years. These 100-kyr-like cycles likely form by bundling of two or three obliquity cycles, indicating that low-latitude Indian-Asian climate variability reflects on increased tilt sensitivity to regional eccentricity insolation changes (pacing tilt cycles) prior to the MPT. The implication of appearance of the 100 kyr cycles in the LR04 and the Fe/K records since the MPT suggests strengthening of a climate link between the low and high latitudes during this period of climate transition. The Correction to this article has been published in Progress in Earth and Planetary Science 2019 6:21 - https://doi.org/10.1186/s40645-019-0259-