Michael T. Ledbetter

Paleoceanography of the Deep Western Boundary Undercurrent on the North American continental margin for the past 25 000 yr

Temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the Deep Western Boundary Undercurrent (DWBUC) on the North American continental margin. From 25 to 17 ka the DWBUC axis was 500 m shallower and farther inshore than its present position on the lower rise; current speed was high from 25 to 20 ka and then waned until 17 ka. Between 17 and 7 ka the major axis of the DWBUC shallowed to 4000 m; current speed increased from 17 to 11 ka but slowed from H to 7 ka. Since 7 ka the DWBUC axis has deepened to its modern position at 4900 m. The zone of maximum carbonate dissolution remained below the DWBUC; the most intense dissolution was from 22 to 9 ka and 5 to 2 ka. Highest abundance of Uvigerina and highest percent benthic foraminifera were immediately above the DWBUC from 22 to 7 ka, after which values of both parameters fell to near zero.

Deep-sea sedimentary processes in the Argentine Basin revealed by high-resolution seismic records (3.5 kHz echograms)

Abstract The regional distribution of nine 3.5 kHz seismic echo types reveals the areal extent of deep-sea sedimentary processes in the Argentine Basin. Correlation between four echo types (IB, IIA, IIB, IIIF) and the relative abundance of sand and silt layers in piston cores shows that the majority of coarse sediment bypasses the continental slope and rise. This coarse sediment is deposited on the proximal abyssal plain with progressively lower amounts reaching basinward across the distal plain. Scarps, irregular and blocky hyperbolic zones, and transparent layers along the Argentine continental margin indicate the widespread occurrence of mass-flow deposits. One large area of mass-flow deposits covers at least 5 × 10 4 km 2 of the continental rise and adjacent abyssal plain east of the Rio de la Plata. A field of large migrating mud waves, approximately 1.0 × 10 6 km 2 in extent, occurs on sediment drifts in the central basin. The mud waves range in amplitude up to 137 m and average 26 m; wavelengths are about 3–7 km. The migrating mud waves are current-controlled features which form in areas of relatively weak Antarctic Bottom Water (AABW) benthic circulation. A variety of hyperbolic echo types on the abyssal plain adjacent to the Argentine continental margin, south of the Rio Grande Rise, and in the Georgia Basin, are also interpreted as AABW current-produced erosional/depositional bedforms, although downslope sedimentary processes may also produce similar hyperbolic echoes. The distribution of bedforms inferred from 3.5 kHz echo character suggests that sediments is supplied to the basin principally by gravity-controlled mass flows. Sediment transported by AABW from higher latitudes is a secondary source. Sediments from both sources is winnowed by strong AABW flow along the Argentine continental rise. The fine-grained component is transported to the central basin where it is deposited as migrating mud waves in regions of relatively weaker AABW flow.

Bottom-current speed in the Vema Channel recorded by particle size of sediment fine-fraction

Abstract The particle-size distribution of the carbonate-free silt fraction was determined in forty-two core-top samples from the east flank of the Vema Channel in order to allow comparison of the mean particle size with near-bottom current speed based on nearby CTD and current-meter observations. The silt mean particle size fluctuates insignificantly between 1450–3950 m but coarsens markedly below 4000 m under Antarctic-source bottom currents. The top of the zone of coarse sizes approximates the top of Lower Circumpolar Water (LCPW) at ∼4000 m; the steep gradient to coarsest sizes in the axis of the channel marks the transition to Antarctic Bottom Water (AABW). The vertical profile of mean particle size on the east flank correlates in general respects with the vertical velocity gradient in the channel axis although the core sites on the east flank are as far as ∼200 miles from the current-meter arrays. Nevertheless, this relationship is used to infer that the non-carbonate particle sizes deposited on the east flank of the channel may be an indication of bottom-current speed and, therefore, the mean size is used to reconstruct profiles of mean current speed for three time slices (18, 120 and 140 ka B.P.). During glacial isotopic stages 2 and 6 the particle sizes deposited on the east flank of the channel are characterized by finer sizes at nearly all depths and the zone of coarse sizes corresponding to LCPW and AABW deepens. During interglacial isotopic stage 5e, the particlesize profile is very similar to the modern profile; however, LCPW and AABW appear to be deeper than at present. The finer particle sizes deposited at nearly all depths during glacial periods indicates that the velocity of deep circulation in the Atlantic was reduced in response to the ice ages. Deep circulation during isotopic stage 5e closely resembled modern circulation; however, AABW production rates may have been less that at present.

Tephrochronology of marine tephra adjacent to Central America

Geochemical analyses performed on volcanic ash layers from the 3 ocean basins surrounding Central America revealed 11 distinct tephra horizons during the past 300,000 yr, and the distribution of each tephra was delineated. The Los Chocoyos Ash (84,000 yr old) and Worzel Layer-L Ash (230,000 yr old) are the 2 most widespread tephra in the region. Two tephra, Y-6 (75,000 yr old) and W-1 (136,000 yr old), are restricted to the western Gulf of Mexico. The remaining tephra—B (36,000 yr old), J 1 (135,000 yr old), G (190,000 yr old), I (212,000 yr old), K (270,000 yr old), I 2 (270,000 yr old), and I 6 (300,000 yr old)—are primarily restricted to the eastern equatorial Pacific Ocean immediately adjacent to Central America. The age of each layer was determined by correlation to oxygen isotope and calcium carbonate stratigraphy in marine cores or by interpolation or extrapolation of ages from that stratigraphy. Of the marine tephra, two were correlated to terrestrial exposures of silicic volcanic rocks. The Layer-D tephra had been correlated to the Los Chocoyos Ash from the Lake Atitlan caldera in Guatemala. The Layer-I 2 tephra is correlated with the T-fall ash from Lake Amatitlan caldera in Guatemala. The age of the terrestrial volcanic units was determined by identifying the stratigraphic age of the marine tephra. The Los Chocoyos Ash and T-fall deposit are dated at 84,000 and ∼270,000 yr B.P., respectively. Geochemical correlation of the marine tephra to other terrestrial sources will facilitate dating of explosive volcanic eruptions in southern Mexico, Central America, and northern South America.

A Late Pleistocene time-series of bottom-current speed in the Vema Channel

Abstract A coarsening of the mean particle size of the carbonate-free silt fraction from sea-floor samples below 4000 m in the Vema Channel has been used to separate high-velocity Antarctic Bottom Water (AABW) from the overlying, slower North Atlantic Deep Water (NADW). A time-series of fluctuations in bottom-current speed within the modern AABW/NADW transition zone was examined by determining the particle-size distribution of sediments from eight gravity cores with a high-resolution stratigraphy for the past 250 kyrs. The bottom-current paleospeed was inferred from a correlation of particle size in seafloor samples with mean current speed from nearby current-meters. The mean bottom-current speed at depths comparable to modern AABW was highest (7–10 cm/s) during interglacial to glacial transitions corresponding to the oxygen isotopic stage 6/7 and 4/5 boundaries and at present. The mean bottom-current speed at depths comparable to modern NADW was nearly uniform for most of the past 250 kyrs except during glacial oxygen isotopic stage 2 when the speed dropped to 2 cm/s, or one-half of the present speed. The application of the “calibrated” particle-size method to examine bottom-current paleospeed allows testing of paleoceanographic models which rely on assumptions or inferences of changes in bottom-water production rate during the late Pleistocene paleoclimatic fluctuations.

Offshore and onshore liquefaction at Moss Landing spit, central California—Result of the October 17,1989, Loma Prieta earthquake

As a result of the October 17, 1989, Loma Prieta (Santa Cruz Mountains, California) earthquake, liquefaction of the fluvial, estuarine, eolian, and beach sediments under a sand spit destroyed the Moss Landing Marine Laboratories and damaged other structures and utilities. Initial studies suggested that the liquefaction was a local phenomenon. More detailed offshore investigations, however, indicate that it occurred over a large area (maximum 8 km{sup 2}) during or shortly after the earthquake with movement of unconsolidated sediment toward and into the head of Monterey submarine canyon. This conclusion is supported by side-scan sonographs, high-resolution seismic-reflection and bathymetric profiles, onshore and sea-floor photographs, and underwater video tapes. Many distinct lobate features were identified on the shallow shelf. These features almost certainly were the result of the October 17 earthquake; they were subsequently destroyed by winter storms. In addition, fresh slump scars and recently dislodged mud debris were found on the upper, southern wall of Monterey submarine canyon.

Tephrochronology of the western Gulf of Mexico for the last 185,000 years

Abstract Tephra in 31 piston cores from the western Gulf of Mexico and 7 piston cores from the equatorial Pacific were analyzed by electron microprobe. Six ash layers in the western Gulf of Mexico were easily distinguished by TiO2, FeO, and CaO contents and correlated by geochemistry in order to determine the distribution pattern for each ash layer. Correlation by geochemistry is an easier, more accurate method than biostratigraphic correlation; some of the tephras were miscorrelated by biostratigraphy. The six tephras were dated by geochemical identification in a piston core with oxygen-isotope stratigraphy and the ages are Y5 (30,000 yr B.P.), Y6 (65,000 yr B.P.), Y8 (84,000 yr B.P.), X2 (110,000 yr B.P.), W1 (136,000 yr B.P.), and W2 (185,000 yr B.P.). Data from this study corroborated correlations of the Y8 tephra in the western Gulf of Mexico with the D layer in the eastern equatorial Pacific Ocean. None of the other five layers in the Gulf of Mexico, however, were found in the Pacific Ocean. The limited distribution of the Y5, Y6, X2, and W2 ash layers close to Mexico indicates possible sources in Mexico. Tephra from the late Pleistocene La Primavera pumice in Mexico, however, does not correlate with the marine tephra.

Influence of bottom currents on sediment texture and sea-floor morphology in the Argentine Basin

Summary The relative bottom-current speed below 4000 m in the Argentine Basin was inferred from the regional pattern of mean particle size of the non-biogenic silt fraction of about 300 sea-floor samples. The pattern reveals a strong deep western boundary current (DWBC) entering the Argentine Basin from the Georgia Basin through a gap in the Falkland Fracture Zone. The DWBC turns W and flows as a contour current along the Falkland Escarpment and Argentine continental margin. At the southern end of the Rio Grande Rise, the DWBC is deflected to the E and SE where it flows along the lower flank of the Mid-Atlantic Ridge as a weak basin-wide return flow. Several smaller return-flow gyres in the southern and western basin are delineated in the regional pattern. The DWBC is strongest along the western margin of the Argentine Basin where 3.5 kHz echograms reveal very prolonged echoes with no sub-bottom reflectors. That reflector pattern is indicative of coarse turbidites, some of which have been winnowed to produce coarse-grained lag deposits under the axis of flow. In the interior of the basin, where bottom-current flow is weakest, sediment consists of clay and silt which is deposited in large migrating mud waves with wavelengths of 3–10 km and heights of up to 137 m (average 26 m). The mud waves consist of material swept into the basin by the DWBC and delivered to the basin margin by down-slope processes. Thus material is winnowed by the DWBC and deposited as a fine-grained chaff in abyssal antidunes which migrate to the centre of the basin and form thick drift deposits.

Late pleistocene to holocene fluctuations in bottom-current speed in the Argentine Basin mudwave field

Abstract Particle-size analyses of core-top samples from traverses across two large asymmetrical mudwaves in the Argentine Basin were used to test a model that predicted slower bottom-current speeds on the steep, upstream (depositional) side and higher speeds on the downstream (erosional or non-depositional) side of the waveform. Consistent with the model, the silt mean particle-size of sediment on the upstream side of both mudwaves was finer than on the downstream side. Downcore analyses of the particle-size of box cores on each side of the two mudwaves were used to identify temporal trends in bottom-current speed. AMS14C dates of box core samples revealed unconformities on the downstream side of each mudwave (22−4.2 kya and 9−4 kya in cores 10 and 24, respectively). Particle sizes in samples above the unconformities were consistently coarser than those on the upstream side of each mudwave, revealing periods of high bottom-current speed from 4 to 2.5 kya. Below the unconformities the period 37 to 22 kya was characterized by high bottom-current speed in core 10 while low speeds were inferred for the period 13−9 kya in core 24. The inferred paleospeed in the cores from the depositional side of the two mudwaves reveals a trend of decreasing speed for the past 3.5 kya with short intervals of increased speed in each core.

Post-Miocene fluctuations of Antarctic Bottom Water paleospeed in the southwest Atlantic Ocean

Abstract A gap in the Falkland Escarpment serves as the primary channel through which Antarctic Bottom Water (AABW) flows into the Argentine Basin from the Georgia Basin. The particle-size distributions of the biogenic-free silt fraction in four cores near the Falkland Gap and the hiatus frequency at water depths greater than 4000 m throughout the Southern Ocean, are used to infer post-Miocene AABW paleospeed. Analysis of AABW paleocirculation indicates episodes of increased bottom-current speeds at: 5.3–4.5; 4.0–3.5; 3.3–3.1; 2.5–2.3; 2.0–1.5; and 1.0 Ma. These fluctuations in inferred paleospeed are compared to both northern and southern hemisphere paleoceanographic and paleoclimatic events in order to examine the response of bottom-water circulation to post-Miocene climates. During the Early Gilbert-Middle Matuyama (5.3–1.5 Ma), southern hemisphere climatic events provided the primary forcing mechanisms on AABW activity. The model invoked involves increased salinity (and overturn) of Weddell Shelf Water in response to increased sea ice formation during cold episodes. From the Late Matuyama (after 1.5 Ma) to present fluctuations in Warm Deep Water resulting from northern hemisphere climatic evolution may have been the predominant factor influencing AABW paleospeed. These episodes of increased bottom-current activity may correlate to periods of mudwave migration in the Argentine Basin during the past 5 Ma. The mudwave fields consist of silty, siliceous clays deposited in response to enhanced sediment supply and fast moving currents in the interior basin. We speculate that the periods of enhanced bottom-water activity may have been responsible for major episodes of mudwave growth that began since the Late Oligocene and have persisted into the Holocene.