Ocean Drilling Program Expedition Research Articles

Mineral Geochemistry of Basaltic Rocks from IODP Expeditions 334 and 344: Implications for Magmatic Processes of Cocos Ridge Segment Being Subducted Beneath the Middle America Trench

The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.

Integrated Pliocene-Pleistocene magnetostratigraphy and tephrostratigraphy of deep-sea sediments at IODP Site U1424 (Yamato Basin, Japan Sea)

Sediments from the semi-enclosed Japan Sea are sensitive to paleoclimatic perturbations and they offer great opportunities for many regional and global paleoceanographic and paleoclimatic studies. These studies often require a robust chronology. However, due to rare preservation of calcareous microfossils and drastic changes in surface water salinity during glacial lowstands, the construction of a traditional oxygen isotope stratigraphy for Japan Sea sediments is often difficult. Here, we use sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1424 to build an integrated Pliocene-Pleistocene reference magnetostratigraphy and tephrostratigraphy for sedimentary sequences from the region. Rock magnetic experiments indicate that magnetic remanence of Site U1424 sediments are carried primarily by (titano)magnetite with small contributions from high coercivity minerals (e.g., hematite) and possibly iron sulphides (pyrrhotite and/or greigite). Dark-colored sediments appear to contain less (titano)magnetite probably due to reductive diagenesis under euxinic conditions. Natural remanent magnetization (NRM) of u-channel samples covering a continuous ~ 167.6 m sediment sequence at Site U1424 was repeatedly measured at 1 cm intervals before and after stepwise demagnetization. Despite lower NRM intensity in dark-colored sediments, NRM directional data from both dark- and light-colored sediments are considered suitable for the construction of magnetostratigraphy. Site U1424 sediments clearly recorded the majority of the polarity chrons and subchrons within the last ~ 4.89 Myr, with the Cobb Mountain subchron, the end of Kaena subchron, and the onset of Nunivak subchron less well preserved. Sixteen tephra layers from the site were sampled for chemical composition analyses and the results were correlated to reference tephrostratigraphy of the region. Ages of the identified tephras are consistent with and can be well integrated with the magnetostratigraphy. The resulting age model suggests that sedimentation rates at Site U1424 range between ~ 1.7 and 7.6 cm/kyr with an average of ~ 3.3 cm/kyr. The acquired magnetostratigraphy and tephrostratigraphy at Site U1424 provide a reference chronology that can be correlated with and transferred to other sediment sequences in the region to study paleoceanographic and paleoclimatic changes of the region as well as their links to other regional and global changes.Graphical abstract

Radiolarian Biostratigraphy and Faunal Turnover across the Early/Middle Miocene Boundary in the Equatorial Pacific

Sedimentary sequences obtained from drilling during Integrated Ocean Drilling Program (IODP) Expedition 320/321, “Pacific Equatorial Age Transect (PEAT)” at eight sites (Sites U1331–1338) in the equatorial Pacific offer an ideal record for reconstructing the evolution of the ocean/climate system throughout the Cenozoic. The sediments drilled at Site U1335 record short-term events of paleoceanographic significance, including the early Miocene climatic optimum (MCO) and the middle Miocene climatic transition (MMCT). Abundant well preserved radiolarians were recovered from the lower Miocene radiolarian Zone RN2 through middle Miocene Zone RN5 at IODP Site U1335. A total of 46 radiolarian datum levels consisting of 20 first occurrences (FOs), 25 last occurrences (LOs), and one evolutionary transition (ET) was recognized within the studied interval at Site U1335. Of these datum levels, 36 radiolarian datum levels were directly tied to the geomagnetic polarity time scale (GPTS) across the early/middle Miocene boundary. The general magnitude of evolutionary change was estimated based on the total turnover rate (the sum of FOs and LOs per 0.5 m.y.) of tropical radiolarians, and two minor faunal turnovers of radiolarian species were recognized between 16.5 and 14.7 Ma and between 13.9 and 13.4 Ma. These faunal turnovers were associated with regional environmental changes such as the increased biological productivity in the equatorial Pacific during the MCO and the MMCT.

Biomass burning history in East Asia during the last 4 million years recorded in elemental carbon variability at IODP site U1423

The burning of trees and grasses produces charred particles, such as charcoal and soot, that can be transported over long distances via winds and rivers to coastal, deltaic, and ocean environments, where they are preserved in sediments. Charcoal contained in sediments has been widely used as a proxy for biomass burning and human activities as well as climate change. Charcoal and soot in Cenozoic marine sediments at Integrated Ocean Drilling Program (IODP) Expedition (Exp.) 346 Site U1423 were measured to examine the regional history of biomass burning in East Asia. Charcoal and soot were measured as elemental carbon (EC) in coarse (> 2 μm) and fine (< 2 μm) fractions using grain size separation by repeated settling followed by application of a thermal optical transmittance (TOT) method. Organic carbon (OC) was also quantified during the process. EC and OC in both coarse and fine fractions are higher from 0 to 1.8 Ma and lower from 1.8 to 4.3 Ma but have large variations, which suggest more frequent or intense biomass burning since 1.8 Ma. Terrestrial biomass and precipitation could be major controls on the EC supply. Fine EC varies independently from coarse EC, which suggests a remote origin of fine EC. Large increases in terrestrial vegetation cover have led to high-temperature burning, which is associated with interglacial stages.

Corganiser: a web-based software tool for planning time-sensitive sampling of whole rounds during scientific drilling

Abstract. Corganiser is a software tool developed to simplify the process of preparing whole-round sampling plans for time-sensitive microbiology and geochemistry sampling during scientific drilling. It was developed during the Integrated Ocean Drilling Program (IODP) Expedition 347, but is designed to work with a wide range of core and section configurations and can thus be used in future drilling projects. Corganiser is written in the Python programming language and is implemented both as a graphical web interface and command-line interface. It can be accessed online at http://130.226.247.137/.

IODP Expedition 336: initiation of long-term coupled microbiological, geochemical, and hydrological experimentation within the seafloor at North Pond, western flank of the Mid-Atlantic Ridge

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 336 addressed questions concerning subseafloor microbial life and its relation to seawater circulation and basalt–seawater reactions in the basaltic ocean crust. Sediment and basement samples were recovered at three drill sites located in the North Pond area, an 8 × 15 km large sediment pond on the 8 Ma western flank of the Mid-Atlantic Ridge around 22°45' N and 46°05' W in roughly 4450 m water depth. The average core recovery rate in basement was approx. 31%. The subseafloor depth of the basement holes ranges from 90 to 332 m; sediment thickness is between 36 and 90 m. Two of the holes (U1382A, and U1383C) were equipped with advanced Circulation Obviation Retrofit Kit (CORK) observatories, employing – for the first time – fiberglass casing. Another CORK string was deployed in Deep Sea Drilling Project (DSDP) Hole 395A, but the wellhead broke off upon final installment. Nonetheless, the North Pond observatory is fully operational and post-cruise observatory research is already underway. Combined geochemical and microbiological studies of the drill core samples and experimental CORK materials will help understand (1) the extent and activity of microbial life in basalt and its relation to basalt alteration by circulating seawater, and (2) the mechanism of microbial inoculation of an isolated sediment pond.

Recent scientific and operational achievements of D/V Chikyu

Abstract The D/V Chikyu, a scientific drilling vessel, is equipped with industry-standard riser capabilities. Riser drilling technology enables remarkable drilling and downhole logging capabilities and provides unprecedented hole-stability, enabling the shipboard team to retrieve high-quality wire-line logging data as well as well-preserved core samples. The 11 March 2011 Tohoku Oki mega-earthquake and tsunami cost over 18,000 casualties in NE Japan. Chikyu, docked in the Port of Hachinohe, was damaged by the tsunami. By April 2012, the ship was back in operation; drilling the toe of the Japan Trench fault zone where topographic surveys suggested there was up to 50 m eastward motion, the largest earthquake rupture ever recorded. During Integrated Ocean Drilling Program (IODP) Expeditions 343 and 343 T, Chikyu drilled 850 m below sea floor (mbsf) in 6,900+ m water depth and recovered core samples of a highly brecciated shear zone composed of pelagic claystone. A subseafloor observatory looking for temperature signatures caused by the fault friction during the earthquake, was installed and later successfully recovered. The recovered temperature loggers recorded data from which the level of friction during the mega-earthquake slip could be determined. Following Exp. 343, Chikyu began IODP Exp. 337, a riser drilling expedition into the Shimokita coal beds off Hachinohe, to study the deep subsurface biosphere in sedimentary units including Paleogene-Neogene coal beds. New records in scientific ocean drilling were achieved in deepest penetration (drilling reached 2,466 mbsf) and sample recovery. Currently Chikyu is conducting deep riser drilling at the Nankai Trough in the final stage of the NanTroSEIZE campaign. During the years 2011 to 2013, including drilling in the Okinawa Hydrothermal System, Chikyu’s operational and scientific achievements have demonstrated that the ship’s capabilities are vital for opening new frontiers in earth and biological sciences.

Species-specific responses of late Miocene Discoaster spp. to enhanced biosilica productivity conditions in the equatorial Pacific and the Mediterranean

Census data of a major Cenozoic calcareous nannofossil genus (Discoaster) have been acquired from Site U1338, located near the Equator in the eastern Pacific Ocean and drilled in 2009 during Integrated Ocean Drilling Program (IODP) Expedition 321. The investigated 147.53 m thick upper Miocene sediment sequence is primarily composed of biogenic carbonate and biogenic silica. Diatom biostratigraphic data were used to develop a revised biomagnetostratigraphic age model, resulting in more variable late Miocene sedimentation rates. Carbonate content variations mainly reflect dilution by biogenic silica production, although intense carbonate dissolution affects a few shorter intervals. Abundance variations of discoasters show no distinct correlation with either carbonate or biosilica contents. The two dominant Discoaster taxa are D. brouweri and D. variabilis, except for a 12 m thick interval where D. bellus outnumbers the sum of all other discoasters by a factor of 4.6. Data presented indicate that first D. hamatus and then D. berggrenii both evolved from D. bellus. Three unusual morphotypes, here referred to as Discoaster A, B and C, increase in relative abundance during episodes of enhanced biosilica production in the upper half of the investigated sequence (Messinian). Strikingly similar morphotypes have been observed previously in Messinian age sediments from the Mediterranean, characterized by alternating deposition of biogenic carbonate and biosilica. This suggests a species-specific response among some of the late Miocene discoasters to broader oceanographic and climatic forcing that promoted episodes of enhanced deposition of biogenic silica.

Scientific Drilling

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution. doi: 10.2204/iodp.sd.15.04.2013

Scientific Drilling

Abstract. The Motion Decoupled Hydraulic Delivery System (MDHDS) is a new downhole tool delivery system that is deployed by wireline and uses drillstring pressure to advance a penetrometer (or other downhole tool) into the formation at the bottom of offshore boreholes. After hydraulic deployment of the penetrometer, it is completely decoupled from the BHA; this eliminates the adverse effects of ship heave. We tested the MDHDS at Site U1402 (the location of Site 1073, ODP Leg 174A), offshore New Jersey, during two days of ship time during Integrated Ocean Drilling Program (IODP) Expedition 342. In one deployment we emplaced a penetrometer successfully and documented that it was decoupled from drillstring movement. Based on this successful field test, the MDHDS has been certified by the U.S. Implementing Organization (USIO) for shipboard use. The MDHDS will replace the previous deployment system, the Colletted Delivery System. The MDHDS is an IODP-funded engineering development led by The University of Texas at Austin, in conjunction with the USIO and Stress Engineering Services. This sea trial was the culmination of a seven-year development effort that included extensive engineering design and fabrication. doi: 10.2204/iodp.sd.15.07.2013

The Motion Decoupled Delivery System: A New Deployment System for Downhole Tools is Tested at the New Jersey Margin

The Motion Decoupled Hydraulic Delivery System (MDHDS) is a new downhole tool delivery system that is deployed by wireline and uses drillstring pressure to advance a penetrometer (or other downhole tool) into the formation at the bottom of offshore boreholes. After hydraulic deployment of the penetrometer, it is completely decoupled from the BHA; this eliminates the adverse effects of ship heave. We tested the MDHDS at Site U1402 (the location of Site 1073, ODP Leg 174A), offshore New Jersey, during two days of ship time during Integrated Ocean Drilling Program (IODP) Expedition 342. In one deployment we emplaced a penetrometer successfully and documented that it was decoupled from drillstring movement. Based on this successful field test, the MDHDS has been certified by the U.S. Implementing Organization (USIO) for shipboard use. The MDHDS will replace the previous deployment system, the Colletted Delivery System. The MDHDS is an IODP-funded engineering development led by The University of Texas at Austin, in conjunction with the USIO and Stress Engineering Services. This sea trial was the culmination of a seven-year development effort that included extensive engineering design and fabrication. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.15.07.2013" target="_blank"&gt;10.2204/iodp.sd.15.07.2013&lt;/a&gt;

IODP Expedition 340T: Borehole Logging at Atlantis Massif Oceanic Core Complex

Integrated Ocean Drilling Program (IODP) Expedition 340T returned to the 1.4-km-deep Hole U1309D at Atlantis Massif to carry out borehole logging including vertical seismic profiling (VSP). Seismic, resistivity, and temperature logs were obtained throughout the geologic section in the footwall of this oceanic core complex. Reliable downhole temperature measurements throughout and the first seismic coverage of the 800–1400 meters below seafloor (mbsf) portion of the section were obtained. Distinct changes in velocity, resistivity, and magnetic susceptibility characterize the boundaries of altered, olivine-rich troctolite intervals within the otherwise dominantly gabbroic se-quence. Some narrow fault zones also are associated with downhole resistivity or velocity excursions. Small deviations in temperature were measured in borehole fluid adjacent to known faults at 750 mbsf and 1100 mbsf. This suggests that flow of seawater remains active along these zones of faulting and rock alteration. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Analysis of wallrock properties, together with alteration and structural characteristics of the cores from Site U1309, highlights the likely interplay between lithology, structure, lithospheric hydration, and core complex evolution. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.15.04.2013" target="_blank"&gt;10.2204/iodp.sd.15.04.2013&lt;/a&gt;

The IODP Expedition 332 : Eyes on the Prism, The NanTroSEIZE Observatories

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major long-term drilling project designed to investigate the seismogenic behavior of subduction zone plate boundaries. Integrated Ocean Drilling Program (IODP) Expedition 332 deployed a long-term borehole monitoring system (LTBMS), an advanced Circulation Obviation Retrofit Kit (CORK)-type observatory. The recovery of pressure and temperature data from a temporary observatory (SmartPlug) deployed during IODP Expedition 319 helped prove the SmartPlug concept. The permanent LTBMS was deployed n the upper 1000 m of Site C0002, while the SmartPlug was recovered from Site C0010 and replaced with a more capable "GeniusPlug", incorporating an extension with a geochem-ical sampler and biological experiment to the original SmartPlug design. SmartPlug pressure and temperature data showed signs of transient pressure events. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.14.04.2012" target="_blank"&gt;10.2204/iodp.sd.14.04.2012&lt;/a&gt;

IODP Expedition 327 and Atlantis Expedition AT 18-07:Observatories and Experiments on the Eastern Flank of the Juan de Fuca Ridge

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 327 (summer 2010) was designed to resolve the nature of fluid-rock interactions in young, upper volcanic crust on the eastern flank of the Juan de Fuca Ridge. Expedition 327 drilled, cased and cored two new basement holes, conducted hydrogeologic experiments, and installed subseafloor borehole observatories (Circulation Obviation Retrofit Kits, CORKs). These CORKs were intended to allow borehole conditions to recover to a more natural state after the dissipation of disturbances caused by drilling, casing, and other operations; provide a long-term monitoring and sampling presence for determining fluid pressure, temperature, composition, and microbiology; and facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes. Expedition 327 was followed (summer 2011) by R/V Atlantis Expedition AT18-07, with the remotely-operated vehicle (ROV) Jason, to service these CORKs, collect subseafloor pressure data, recover and deploy autonomous fluid and microbial samplers, collect large volumes of borehole fluids, and initiate a cross-hole hydrogeologic experiment using an electromagnetic flow meter. In addition, Atlantis Expedition AT18-07 refurbished an old CORK that could not be replaced during IODP Expedition 327, completing a critical part of the three-dimensional observation network that is currently being used to monitor a large-scale, directional formation response to long-term fluid flow from the crust. doi: 10.2204/iodp.sd.13.01.2011

IODP Expedition 324: Ocean Drilling at Shatsky Rise Gives Clues about Oceanic Plateau Formation

Abstract. Integrated Ocean Drilling Program (IODP) Expedition 324 cored Shatsky Rise at five sites (U1346–U1350) to study processes of oceanic plateau formation and evolution. Site penetrations ranged from 191.8 m to 324.1 m with coring of 52.6 m to 172.7 m into igneous basement at four of the sites. Average recovery in basement was 38.7%–67.4%. Cored igneous sections consist mainly of variably evolved tholeiitic basalts emplaced as pillows or massive flows. Massive flows are thickest and make up the largest percentage of section on the largest and oldest volcano, late Jurassic age Tamu Massif; thus, it may have formed at high effusion rates. Such massive flows are characteristic of flood basalts, and similar flows were cored at Ontong Java Plateau. Indeed, the similarity of igneous sections at Site U1347 with that cored on Ontong Java Plateau implies similar volcanic styles for these two plateaus. On younger, smaller Shatsky Rise volcanoes, pillow flows are common and massive flows thinner and fewer, which might mean volcanism waned with time. Cored sediments from summit sites contain fossils and structures implying shallow water depths or emergence at the time of eruption and normal subsidence since. Summit sites also show pervasive alteration that could be due to high fluid fluxes. A thick section of volcaniclastics cored on Tamu Massif suggests that shallow, explosive submarine volcanism played a significant role in the geologic development of the plateau summit. Expedition 324 results imply that Shatsky Rise began with massive eruptions forming a huge volcano and that subsequent eruptions waned in intensity, forming volcanoes that are large, but which did not erupt with unusually high effusion rates. Similarities of cored sections on Tamu Massif with those of Ontong Java Plateau indicate that these oceanic plateaus formed in similar fashion. doi: 10.2204/iodp.sd.12.03.2011

Towards ground truthing exploration in the central Arctic Ocean: a Cenozoic compaction history from the Lomonosov Ridge

ABSTRACTThe Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), recovered the first Cenozoic sedimentary sequence from the central Arctic Ocean. ACEX provided ground truth for basin scale geophysical interpretations and for guiding future exploration targets in this largely unexplored ocean basin. Here, we present results from a series of consolidation tests used to characterize sediment compressibility and permeability and integrate these with high‐resolution measurements of bulk density, porosity and shear strength to investigate the stress history and the nature of prominent lithostratigraphic and seismostratigraphic boundaries in the ACEX record. Despite moderate sedimentation rates (10–30 m Myr−1) and high permeability values (10−15–10−18 m2), consolidation and shear strength measurements both suggest an overall state of underconsolidation or overpressure. One‐dimensional compaction modelling shows that to maintain such excess pore pressures, an in situ fluid source is required that exceeds the rate of fluid expulsion generated by mechanical compaction alone. Geochemical and sedimentological evidence is presented that identifies the Opal A–C/T transformation of biosiliceous rich sediments as a potential additional in situ fluid source. However, the combined rate of chemical and mechanical compaction remain too low to fully account for the observed pore pressure gradients, implying an additional diagenetic fluid source from within or below the recovered Cenozoic sediments from ACEX. Recognition of the Opal A–C/T reaction front in the ACEX record has broad reaching regional implications on slope stability and subsurface pressure evolution, and provides an important consideration for interpreting and correlating the spatially limited seismic data from the Arctic Ocean.

Stable Carbon Isotopic Response of the Benthic Foraminifera from IODP 311 to the Marine Methane Hydrate Geo-system

A preliminary research of the benthic Foraminifera from Integrated Ocean Drilling Program(IODP) Expedition 311 has been carried out for the purpose of better understanding their stable carbon isotopic response to the marine gas hydrate geo-system.SEM photographs and stable oxygen isotopes of the species Uvigerina peregrina and Bulimina mexicana indicated that diagenesis and authigenic carbonate precipitation had little effect on Foraminifera.Uvigerina peregrina and Bulimina mexicana collected from the sediments in seepage Site U1328 and in adjacent non-seepage Site U1327 exhibited a distinct negative δ13C excursion with values of U.peregrina-0.67‰～-2.75‰ PDB,B.mexicana-0.51‰～-1.52‰ PDB in Site U1327 and U.peregrina-0.72‰～-2.71‰ PDB,B.mexicana-0.58‰～-1.45‰ PDB in Site U1328 respectively.The formation of distinct stable carbon isotopic excursions of benthic Foraminifera could be most likely interpreted by the involvement of both anaerobic oxidation of methane(AOM) and food sources in sediments.The distinct negative carbon isotopic excursion of benthic Foraminifera thus could be believed as one of indicators to recognize the marine gas hydrate geo-system.

Constraints on the Pleistocene chronology of sediments from the Lomonosov Ridge

Despite its importance in the global climate system, age‐calibrated marine geologic records reflecting the evolution of glacial cycles through the Pleistocene are largely absent from the central Arctic Ocean. This is especially true for sediments older than 200 ka. Three sites cored during the Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), provide a 27 m continuous sedimentary section from the Lomonosov Ridge in the central Arctic Ocean. Two key biostratigraphic datums and constraints from the magnetic inclination data are used to anchor the chronology of these sediments back to the base of the Cobb Mountain subchron (1215 ka). Beyond 1215 ka, two best fitting geomagnetic models are used to investigate the nature of cyclostratigraphic change. Within this chronology we show that bulk and mineral magnetic properties of the sediments vary on predicted Milankovitch frequencies. These cyclic variations record “glacial” and “interglacial” modes of sediment deposition on the Lomonosov Ridge as evident in studies of ice‐rafted debris and stable isotopic and faunal assemblages for the last two glacial cycles and were used to tune the age model. Potential errors, which largely arise from uncertainties in the nature of downhole paleomagnetic variability, and the choice of a tuning target are handled by defining an error envelope that is based on the best fitting cyclostratigraphic and geomagnetic solutions.