ALBERT

Description: Multibeam bathymetry and 3.5-kHz sub-bottom profiler data collected from the US icebreaker Healy in 2003 provide convincing evidence for grounded ice on the Chukchi Borderland off the northern Alaskan margin, Arctic Ocean. The data show parallel, glacially induced seafloor scours, or grooves, and intervening ridges that reach widths of 1000 m (rim to rim) and as much as 40 m relief. Following previous authors, we refer to these features as “megascale glacial lineations (MSGLs).” Additional support for ice grounding is apparent from stratigraphic unconformities, interpreted to have been caused by ice-induced erosion. Most likely, the observed sea-floor features represent evidence for massive ice-shelf grounding. The general ESE/WNW direction of the MSGLs, together with sediment, evidently bulldozed off the Chukchi Plateau, that is mapped on the western (Siberian) side of the plateau, suggests ice flow from the Canada Basin side of Chukchi Borderland. Two separate generations of glacially derived MSGLs are identified on the Chukchi Borderland from the Healy geophysical data. The deepest and oldest extensive MSGLs appear to be draped by sediments less than 5 m thick, whereas no sediment drape can be distinguished within the resolution of the sub-bottom profiles on the younger generation.

Description: Conventional and accelerator mass spectrometry 14C ages indicate that laminated sediment in three cores from the northern California continental slope near 38°N and 39°N were deposited between 42,000 and 25,000 yr B.P. This revises and refines our previous estimates that laminated sediment accumulated during the late Pleistocene to early Holocene (J. V. Gardner and E. Hemphill-Haley, 1986, Geology 14, 691-694). Preservation of laminated sediment on the upper slope in this area suggests a period of intense coastal upwelling, high primary productivity, and resultant depletion of oxygen in bottomwaters preceding the onset of global glacial conditions. The transition from Pleistocene to Holocene conditions, and the establishment of a modern climatic regime driven by the California Current, included the incursion of the subtropical diatom, Pseudoeunotia doliola. P. doliola is common in sediment younger than about 10,000 yr and thus is a reliable marker species for identifying Holocene deposits off northern California.

Description: Many cores from the deep basins of the Bering Sea have a thin oxidized zone within otherwise reduced sediment. This oxidized zone began to form about 6000 yr ago and represents an interval of about 3200 yr. Mineralogically, the oxidized and reduced sediments are similar, but chemically they differ. Concentrations of Fe and C are lower, and concentrations of Mn, Ba, Co, Mo, and Ni are higher in the oxidized than in the reduced sediment. Mn is enriched about 10-fold in the oxidized zone relative to its concentration in the reduced sediment, Mo about threefold, and Ba, Co, and Ni about twofold. These data suggest that the oxidized zone developed diagenetically as the result of the balance between the flux of organic matter and the available dissolved oxygen in bottom and interstitial waters.We propose that the Bering Sea was substantially ice covered when global glacial conditions prevailed. during the transition to global interglacial conditions, seasonal meltwater from thawing sea ice formed a lens of fresh water that decreased organic productivity. During the winter seasons, however, sea ice reformed and caused downwelling of dense, oxygen-rich waters to recharge bottom waters. The combination of lower organic productivity and more oxygen-rich bottom water allowed oxidized sediment to accumulate. Once full interglacial conditions were established, the volume of sea ice produced was insufficient to affect either productivity or the supply of dissolved oxygen and so bottom conditions again became reducing.Similar events probably occurred during the onset of global glacial conditions, and similar oxidized layers probably formed at these times. Such oxidized zones are highly unstable, however, in a reducing environment and, once buried beyond the influence of bacterial and infaunal activities, are depleted of their available oxygen and converted to reduced sediment.

Description: Seasonal production of siliceous phytoplankton and a seasonal flux of terrigenous clastic material produced varved sediments along the continental slope off California during the late Pleistocene to mid-Holocene. Light-colored sediment within laminations and in sediment-trap samples was deposited during summer upwelling and contains an abundance of the diatoms Skeletonema costatum and Chaetoceros spp. resting spores. Dark-colored sediment deposited in the fall and winter contains abundant Thalassiosira pacifica, and has more terrigenous material. Distribution of diatoms in varves shows that seasonal upwelling has persisted along the California coast and has remained strongly seasonal since the late Pleistocene.

Description: Intensification of the North and South Equatorial Current systems and trade winds occurred during glacial periods, according to a comparison of late Holocene (interglacial), 18,000 B.P. (glacial), and late Quaternary (0 to 180,000 B.P.) faunal assemblages and sea-surface temperature estimates from the equatorial Atlantic and Caribbean regions. Faster circulation of the North Equatorial Current system in glacial Northern Hemisphere winters (February) is indicated by increased upwelling of cool (15°C) water off northwest Africa and slightly cooler conditions across the northern tropical Atlantic and Caribbean. Intensification of the South Equatorial Current occurred along the Equator during the Southern Hemisphere winter (August). This interpretation is based on the dominance of a cool-equatorial assemblage, which indicated that waters of 16° to 18°C replaced the tropical assemblage that lives today in 24° to 26°C water in this region. The cool influence of the glacial (August) Benguela-South Equatorial Current decreased rapidly westward along the equatorial belt so that the fauna was dominated by the tropical assemblage in the Caribbean. Sea-surface temperatures increased rapidly from east to west in the equatorial belt, so that at long 35°W, the 16°C water had reached ambient temperatures of 24° to 26°C. Both faunal assemblages and temperature estimates of eight late Quaternary Atlantic and Caribbean sediment cores show that the equatorial region experienced three maximum incursions of cool Benguela Current water during the past 150,000 yr—at approximately 135,000 B.P., 73,000 B.P., and 18,000 B.P. Differences of glacial to interglacial sea-surface temperatures range from 5° to 10°C in the eastern equatorial Atlantic to 2° to 3°C in the western Atlantic and the Caribbean. During this time, only two periods with similar faunas and surface temperatures occurred—today and 125,000 B.P. Seasonal temperature contrast (August to February) is three to four times greater in all cores for glacial conditions than for interglacial conditions. The winter temperatures (February to the north of the thermal equator and August south of it) show the greatest changes, and they control the overall temperature pattern. Identical temperature patterns for cores affected by the North and the South Equatorial Currents suggest that the Northern and Southern Hemispheres are generally in phase and that more severe winters control the glacial temperature pattern.