Search for: All records

A gas hydrate assessment at International Ocean Discovery Program Expedition 400 drill sites was conducted using downhole logging and core data. Here, we calculate and present the base of gas hydrate stability zone at Expedition 400 drill sites in Baffin Bay, northwest Greenland. We used data from downhole logs and sediment cores from Sites U1603, U1604, U1607, and U1608 to assess hydrate and did not find evidence for the presence of hydrate. At Site U1606, only core data were acquired that showed a decrease in pore water salinity, potentially indicating the presence of hydrate; however, further confirmation was not possible due to the unavailability of downhole logging data. Because of the limitation of the acquired data at the drill sites, a further assessment to confirm the presence of hydrate was not possible. Although hydrate was not identified at any drill sites, hydrate might still be present in the region. 

The procedures and tools employed in coring operations and in the various shipboard laboratories of the R/V JOIDES Resolution are documented here for International Ocean Discovery Program (IODP) Expedition 400. This information applies only to shipboard work described in the Expedition reports section of the Expedition 400 Proceedings of the International Ocean Discovery Program volume. Methods for shore-based analyses of Expedition 400 samples and data will be described in separate individual publications. This introductory chapter describes the procedures and equipment used for drilling, coring, core handling, and sample registration; the computation of depth for samples and measurements; and the sequence of shipboard analyses. Subsequent sections describe laboratory procedures and instruments in more detail. 

Site U1607 (proposed Site MB-07B), the easternmost site in the Expedition 400 site transect, was cored at 74°29.5499′N, 60°34.9900′W at 739 meters below sea level (mbsl) on the middle shelf (Figure F1). Extensive seismic and limited borehole data indicate that this site captures Megaunits C, D1, and D2, interpreted as a middle–late Miocene sediment drift that overlies a succession of mainly hemipelagic strata, possibly of early Miocene to Oligocene age (Knutz et al., 2022b) (Figures F2, F3). Accordingly, Site U1607 may capture the time period from 6 to 30 Ma. 

Site U1604 (proposed Site MB-02C) was cored on the lower slope below the Melville Bugt Trough Mouth Fan (TMF) at 1943 meters below sea level (mbsl) at 73°06.9077′N, 63°47.3996′W below a prominent crescent-shaped protrusion of the Melville Bugt TMF on the northwest Greenland shelf margin (Figure F1). The site was aimed at retrieving a continuous high-resolution record of ice sheet–ocean interactions and processes going back to early Pleistocene. The depth target was the base of an expanded drift channel succession situated on the lower slope between two paleo–ice sheet outlets representing major drainage routes for the northern Greenland ice sheet (NGrIS) into Baffin Bay (Knutz et al., 2019; Newton et al., 2017, 2020, 2021). The sedimentary succession is covered by regional 2D industry data as well as high-resolution multichannel seismic data providing a detailed seismic stratigraphy to guide the drilling objectives (Figure F2). The strategy at Site U1604 was to core through the youngest seismic units (9–11) to capture the expanded depositional sequence within Seismic Unit 8 showing stratified intervals with asymmetric geometries resembling contourite drifts. Site U1604 is paired seismic-stratigraphically with Site U1603, located 16 nmi (~30 km) to the southeast adjacent to a deepwater channel. With Horizon 7 as a stratigraphic target (about 420 meters below seafloor [mbsf]), the succession cored at Site U1604 is complementary to Site U1603 but records sedimentation that is more distal to the channel system and presumably recovers an interval older than at Site U1603. 

Site U1605 (proposed Site MB-31A) was cored at 73°33.6421′N, 62°09.0687′W at 529 meters below sea level (mbsl) on the outermost part of the northwest Greenland shelf margin into Baffin Bay (Figure F1). Constrained by high-quality seismic data, the main objective was to recover potential marine intervals within packages of flat-lying, semicontinuous reflections developed between strong reflections that are interpreted as glacial unconformities within the topset strata succession of the Melville Bugt Trough Mouth Fan (Figure F2) (Knutz et al., 2019; Newton et al., 2021). Site U1605 captures Seismic Units 7–10 with a target depth just below Seismic Horizon 6, corresponding to 282 meters below seafloor (mbsf; based on an average P-wave velocity of 2200 m/s). The sequence is considered to extend to Early Pleistocene age and thus overlaps seismic-stratigraphically with deepwater Sites U1603 and U1604. The primary lithology was assumed to be compacted diamicton with intervals of sand and pebbly mud and possibly marine to glaciomarine deposits (Figure F3). As such, low but variable recoveries were expected at Site U1605. 

Elucidating the geologic history of the Greenland ice sheet (GrIS) is essential for understanding glacial instability thresholds, identified as major climate system tipping points, and how the cryosphere will respond to anthropogenic greenhouse gas emissions. To address current knowledge gaps in the evolution and variability of the GrIS and its role in Earth’s climate system, International Ocean Discovery Program (IODP) Expedition 400 obtained sedimentary records from Sites U1603–U1608 across the northwest Greenland margin into Baffin Bay where thick Cenozoic sedimentary successions can be directly linked to the evolution of the northern GrIS (NGrIS). The strategy of drilling along this transect was to retrieve a composite stratigraphic succession representing the late Cenozoic era from the Oligocene/early Miocene to the Holocene. The proposed sites targeted high–accumulation rate deposits associated with contourite drifts and potential interglacial deposits within a trough mouth fan system densely covered by seismic data. The principal objectives were to (1) test if the NGrIS underwent near-complete deglaciations in the Pleistocene and assess the ice sheet’s response to changes in orbital cyclicities through the mid-Pleistocene transition, (2) ascertain the timing of the NGrIS expansion and examine a hypothesized linkage between marine heat transport through Baffin Bay and high Arctic warmth during the Pliocene, and (3) provide new understandings of climate-ecosystem conditions in Greenland during the geologic periods with increased atmospheric CO2 compared to preindustrial values, encompassing the last 30 My. The deep time objective was attained by coring at Site U1607 on the inner shelf to 978 meters below seafloor, capturing a succession of mainly Miocene and Oligocene age. The six sites drilled during Expedition 400 resulted in 2299 m of recovered core material, and wireline downhole logging was completed at Sites U1603, U1604, U1607, and U1608. This unique archive will provide the basis for understanding the full range of forcings and feedbacks—oceanic, atmospheric, orbital, and tectonic—that influence the GrIS over a range of timescales, as well as conditions prevailing at the time of glacial inception and deglacial to interglacial periods. We anticipate that the shipboard data and further analytical work on Expedition 400 material can constrain predictive models addressing the GrIS response to global warming and its impending effects on global sea levels. 

Site U1608 (proposed Site MB-06D) was cored at 74°7.6818′N, 60°58.3172′W at 607 meters below sea level (mbsl) on the middle section of the northwest Greenland shelf, west of the Melville Bay Ridge and graben structures formed during Cretaceous rifting (Figure F1; see Figure F4 in the Expedition 400 summary chapter [Knutz et al., 2025b]). The main coring targets are mounded contourite drift deposits of expected Pliocene age associated with Megaunit B (Knutz et al., 2015, 2019; Aubrey et al., 2021) and overlying sediments of Megaunit A, recording the transition into glacigenic deposits of earliest trough mouth fan (TMF) progradation (Figure F2). The expanded interval of Megaunit B, captured at Sites U1606 and U1608, reflects deposition below a major incised escarpment that is at least 500 m tall and extends into disturbed sediment packages interpreted as mass transport deposits (Figure F3). The base of the contourite drift accumulation is defined by Horizon c1 of probable Late Miocene age (Knutz et al., 2015). Site U1608 ends ~100 m above Horizon c1, which at this location is characterized by an erosional unconformity related to the slope instability that is strongly expressed in the seismic record (Figure F3). In the context of the full development of the Melville Bugt TMF, Site U1608 targeted TMF Seismic Unit 1, which records the first advance of the northwestern Greenland ice sheet (GrIS) onto the continental shelf, which is hypothesized to correspond to the Pleistocene/Pliocene boundary (Knutz et al., 2019) (Figures F2, F3). Combined, the cores from middle shelf Sites U1606 and U1608 access archives of ocean and climate conditions presumably much warmer than today that were buried by glacial deposits representing global cooling and expansion of northern hemisphere glaciers. These sites use the dynamic drift morphology to capture different parts of Megaunit B strata to capture high-resolution records of the Pliocene ocean-climate system at high Arctic latitudes. 

Site U1606 (proposed Site MB-17A) was cored at 74°13.9380′N, 61°2.2426′W at 653 meters below sea level (mbsl) on the middle section of the northwest Greenland shelf (Figure F1). The site targets the transition from preglacial contourite drift sediments into glacigenic deposits of the earliest trough mouth fan progradation (Seismic Unit 1), marking the first advance of the northern Greenland ice sheet onto the continental margin (Figure F2). The drilling target was at 411 m core depth below seafloor, Method A (CSF-A), near the base of Megaunit B above Horizon c1 (Figure F3), coring a succession that is complementary to Site U1608. The uppermost 180 m at Site U1606 targets a geographically restricted depositional unit, possibly an erosional remnant, aimed at capturing a stratigraphic record of the preglacial to glacial transition that hypothetically corresponds to the Pleistocene/Pliocene boundary (Knutz et al., 2019). Below 150–180 m CSF-A, Site U1606 targets a 200 m thick sedimentary unit characterized internally by a uniform bundle of tilting strata that converges updip against a fault defining an erosional scarp above Horizon c1 (Figure F3). The seismic geometries imply accumulation of sedimentary drift deposits of Megaunit B over a significantly truncated section of Megaunit C (Knutz et al., 2015). The lower sedimentary unit of Site U1606 has a corresponding section at the base of Site U1608 (Figures F2, F3). 

Site U1603 (proposed Site MB-23A) is located at 72°59.04′N, 62°58.83′W in Baffin Bay at 1801 meters below sea level (mbsl), below a protrusion of the northwest Greenland margin shaped by the Melville Bugt Trough Mouth Fan (TMF) (Figure F1). The site targets an expanded drift-channel succession situated on the lower slope between two paleo–ice sheet outlets that represent major drainage routes for the northern Greenland ice sheet (NGrIS) into Baffin Bay (Knutz et al., 2019; Newton et al., 2017, 2021). The drilling strategy was to core a continuous high-resolution record of ice-ocean processes spanning the early late Pleistocene. 

We develop amortized population Gibbs (APG) samplers, a class of scalable methods that frame structured variational inference as adaptive importance sampling. APG samplers construct high-dimensional proposals by iterating over updates to lower-dimensional blocks of variables. We train each conditional proposal by minimizing the inclusive KL divergence with respect to the conditional posterior. To appropriately account for the size of the input data, we develop a new parameterization in terms of neural sufficient statistics. Experiments show that APG samplers can be used to train highly-structured deep generative models in an unsupervised manner, and achieve substantial improvements in inference accuracy relative to standard autoencoding variational methods.