Clark Sherman

Stratigraphic constraints on the timing and emplacement of the Alika 2 giant Hawaiian submarine landslide

Abstract Previous work has found evidence for giant tsunami waves that impacted the coasts of Lanai, Molokai and other southern Hawaiian Islands, tentatively dated at 100+ and 200+ ka by U-series methods on uplifted coral clasts. Seafloor imaging and related work off Hawaii Island has suggested the Alika phase 2 debris avalanche as the source of the ∼100 ka “giant wave deposits”, although its precise age has been elusive. More recently, a basaltic sand bed in ODP site 842 (∼300 km west of Hawaii) estimated at 100±20 ka has been suggested to correlate with this or another large Hawaiian landslide. Our approach to the timing and linkage of giant submarine landslides and paleo-tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature, beginning with a suite of seven piston, gravity and box cores collected in the vicinity of the Alika 2 slide. We used U-series dating techniques, including excess 230 Th and 210 Pb profiling, high-resolution paleomagnetic stratigraphy, including continuous, U-channel analysis, δ 18 O stratigraphy, visual and X-ray sediment lithology, and the petrology and geochemistry of the included turbidites and ash layers. Minimum ages for the Alika phase 2a slide from detailed investigation of two of the cores are 112±15 ka and 125±24 ka (2σ) based on excess 230 Th dating. A less precise age for the Alika phase 1 and/or South Kona slide is 242±80 ka (2σ), consistent with previous geological estimates. Oxygen isotope analyses of entrained planktonic foraminifera better constrain the Alika phase 2a maximum age at 127±5 ka, which corresponds to the beginning of the stage 5e interglacial period. It is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers. Our study indicates the contemporaneity of the Alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on Lanai and Molokai through direct dating and compositional analysis of the landslide deposits.

New evidence for two highstands of the sea during the last interglacial, oxygen isotope substage 5e

Sedimentologic, stratigraphic, and geochronologic analyses of a previously undescribed carbonate section on Oahu, Hawaii, provide new evidence for two distinct sea-level highstands on Oahu during the last interglacial period (oxygen isotope substage 5e). Whereas electron-spin-resonance and uranium-series ages (122 ±8 ka to 152 ±25 ka, and 115 ±10 ka to 160 ±15 ka, respectively) of in situ corals place the age of the deposits within substage 5e, it is the unique sequence of strata found in these exposures that reveals the two transgressions. A highstand lagoonal deposit of coral-algal bafflestone is overlain by large seaward-dipping slabs of beach-rock. The beachrock, deposited during a mid-5e regression, is in turn overlain by a second highstand lagoonal deposit. This sequence was deposited in a broad, shallow, back-reef embayment that was very sensitive to fluctuations in sea level. Elsewhere, along much of the shoreline of Oahu, an in situ coral-algal framestone (Waimanalo Formation), representing the initial 5e highstand, is erosionally truncated on its upper surface. This erosional unconformity represents the mid-5e lowstand and separates the framestone from overlying, seaward-dipping, planar-bedded grainstone and rudstone (Leahi Formation) that accumulated during the second 5e highstand.

Mesophotic coral ecosystems under anthropogenic stress: a case study at Ponce, Puerto Rico

Mesophotic coral ecosystems (MCEs) were compared between La Parguera and Ponce, off the south coast of Puerto Rico. In contrast to La Parguera, Ponce has a narrow insular shelf and hosts several river outlets, a commercial port, a regional sewage treatment plant with associated deep water outfall, and three deep dredge disposal sites. Off Ponce, MCEs receive higher (16×) rates of sedimentation than off La Parguera, a less impacted site. The most impacted sites were located offshore of Cayo Ratones and are in or down-current and in close proximity to one of the dredge disposal sites. There, MCEs are characterized by a steep, irregular, rocky slope with a cover of fine-grained, dark brown sediment, which increases with depth. At shallower depths, scattered rocky outcroppings are colonized by sponges, black corals and algae. The sediment cover contains two to three times the terrigenous content and a significantly higher percentage of the fine-grained fraction than off La Parguera. Thirteen remotely operated vehicle (ROV) dives east and west of Ponce showed that the deepest depth at which corals were observed increased with distance from Cayo Ratones and did not approach depths observed off La Parguera except at the eastern-most (up-current) site, Caja de Muertos, which was also significantly further offshore. Benthic communities off Caja de Muertos were comparable to those at La Parguera, while off Cayo Ratones, there were no mesophotic corals and sparse development of other benthic macrobiota except sponges. Management authorities should include MCEs when assessing potential impacts from anthropogenic activities and take the necessary steps to reduce local threats.

Spatial and temporal patterns in reef sediment accumulation and composition, southwestern insular shelf of Puerto Rico

Abstract. Effects of terrigenous sedimentation are considered a serious threat to Puerto Ricos coral reefs. This study assesses: 1) the composition of sediments accumulating at reef sites on the southwestern shelf of Puerto Rico; 2) the spatial extent to which terrigenous materials are reaching these reefs; and 3) the spatial and temporal variability of sediment composition, grain size and trap collection rates. Sediment traps were deployed at 9 sites from inner shelf to shelf edge. Analyses of total sediment weight (April 2006 – April 2007) show a similar temporal pattern at all sites with June and August having the highest accumulation rates. In general, there is a decrease in trap accumulation rate with depth. Carbon composition analyses indicate that samples consist primarily of calcium carbonate with lesser amounts of terrigenous and organic material, and a slightly higher percentage of terrigenous material in the <63 µm fraction. At a given site, the percentage of terrigenous material is fairly constant regardless of trap collection rate, suggesting that observed changes in accumulation rates are due to resuspension of existing material rather than an influx of new terrigenous material. These results, illustrate that in some reef systems, resuspension of existing bottom sediments is as important as the influx of new terrigenous material.

Coral growth rates from the Holocene Cañada Honda fossil reef, Southwestern Dominican Republic: Comparisons with modern counterparts in high sedimentation settings

Abstract. The Holocene Cañada Honda fossil reef, located in southwestern Dominican Republic, provides a unique opportunity to examine a well-preserved fossil coral reef that thrived in a high-sedimentation environment between 9,000 to 5,000 years ago. Measurements of coral growth rates from the corals Montastraea faveolata and Siderastrea siderea were conducted and comparisons made with growth rate data of these same species from modern coral reefs throughout the Caribbean. Also, assessments of coral species abundance, morphology, age, and distribution, as well as reef sediment composition, were made to determine the paleoenvironment of reef accretion. This reef is characterized by a high relative abundance of sediment-tolerant coral species that have a tendency to form almost monospecific stands. Individual colonies have a propensity to grow as encrusting, dome-shaped, platy-like forms and specimens of Montastraea faveolata commonly contain bands of sediment incorporated into the skeleton. Calibrated radiocarbon ages of fossil corals range from 9,256±137 to 6,737±94.5 BP. Correlation of radiocarbon ages with well-established Holocene sea-level curves indicates that most corals on this reef developed at depths >15m. Measured growth rates in Siderastrea siderea (0.2–0.4 cm/yr) and Montastraea faveolata (0.09–0.44 cm/yr) are relatively low compared with growth rates from modern reef sites, indicating reduced light intensity caused by coral growth at depths greater than 15 m. Reef sediment is characterized by more than 85% carbonate material. A significant portion of the carbonate is allochtonous and was derived from nearby Neogene limestones. The reef was able to survive under high-sedimentation conditions because the high carbonate content of incoming terrigenous sediment would have allowed better light penetration and probable sporadic storms would provide intervening low-sedimentation periods during which reef corals could respond and grow back, keeping-up with sedimentation.