Geoscience professor Steve Hovan coauthored a presentation entitled “Sediments on the Line Islands Ridge Provides a Suitable Archive for Paleoceanographic Research.”
His report was based on research results stemming from the summer 2012 oceanographic expedition to the Line Island Ridge (central Pacific). This presentation was at the American Geophysical Union’s fall 2012 meeting, attended by more than 20,000 earth scientists from around the world.
The central tropical Pacific is an ideal place to monitor changes in the Pacific Marine ITCZ, ENSO, and changes in equatorial productivity and upwelling. Previous work in Pleistocene and Holocene sediment in the Central Tropical Pacific has been hampered by carbonate dissolution and poor time resolution due to the depth of the sea floor at or below the carbonate compensation depth in this region. The Line Islands Ridge is a broad, relatively shallow feature spanning a latitude range of 0 to 10°N and topped in many areas with carbonate sediments.
During a cruise to this feature in May 2012 (MGL12-08), we surveyed selected areas of the ridge that appeared promising for collecting sediment cores using multibeam bathymetry/backscatter, 3.5 kHz single channel seismic reflection, and multi-channel seismic reflection surveys. Above 2,000 m water depth, the plateau is covered by mostly smooth sediments, cut by an occasional dendritic channel. In some places the sediments were as thick as 1.2 km, but volcanic features also protruded through the sediment surface. Evidence for strong current activity included the smooth surfaces, sediment waves, and large sediment drifts in the lee of volcanic features. Attempts to core these sediments suggested that these sediments are composed primarily of winnowed foraminifera sands.
The sediments below 2,000 m water are typically marked by erosion. The style of erosion varied, from gently stepping incised terraces near the equator to more mature looking dendritic channels further north. In general, the ridge tops between channels showed low backscatter on the multibeam suggesting recent sedimentation, and the sides and bottoms of the channels showed higher backscatter suggesting sandy/harder ground. Multi-channel seismic reflection surveys across the channels show them to be eroded into older sediment. Coring sites were chosen at water depths of 2,500–3,500 m on local highs or ridges in between the more deeply eroded channels. Multi-cores, gravity cores, and piston cores were collected. Sediments consist mainly of carbonate oozes dominated by foraminiferal and nanofosssil components.
The cores were analyzed on board with a multi-sensor track system. Regionally coherent signals in sediment density allow for stratigraphic correlation between core sites. Post-cruise oxygen isotope measurements on G. ruber foraminifera from selected cores indicate that most of the cores will provide suitable material for Late Quaternary paleoceanographic resconstructions. Sedimentation rates range from 3.5 cm per thousand years near the equator to 1.5 cm per thousand years at 6°N.
Department of Geoscience