Miocene through Pleistocene dinoflagellates from ODP Site 170-1039

Palynological data from offshore Costa Rica, allow us to investigate the relationship between dinoflagellate cyst assemblages and changes in regional oceanic primary productivity. From Miocene to Pleistocene, productivity at ODP Site 1039 was influenced by tectonic drift, as Site 1039 approached the continent, from the Equator to its current position at ~10°N. In addition, dinoflagellate abundance is modulated by regional productivity events, which modified primary productivity, as also indicated by available data on calcareous nannofossils, diatoms, TOC, and CaCO3 content. Five palynomorph intervals are defined. The early-late Miocene one, dominated by Batiacasphaera, represents relatively stable, productive oceanic conditions before the closure of the Indonesian and Panama Seaways. The late Miocene decrease in palynomorph recovery is related to the Carbonate Crash Event. The high abundance and diversity of the assemblages at the end of the late Miocene to early Pliocene indicate increased productivity related to the Global Biogenic Bloom, and a change in dominance from Batiacasphaera to Impagidinium to Nematosphaeropsis. The low abundance of the late Pliocene interval is related to El Niño-like conditions, and there is another change related to the disappearance of Batiacasphaera and dominance of Impagidinium, Nematosphaeropsis, and Operculodinium. The abundant Pleistocene assemblages represent increased marine productivity, and a high influx of continental palynomorphs and bissacate pollen, associated with the proximity of the Costa Rica Dome. Pleistocene dinoflagellates are characterized by Spiniferites and Selenopemphix, together with rare Impagidinium and Nematosphaeropsis.

Barium in sediments and pore waters of several ODP Leg 170 and Leg 205 sites

Barium concentrations were measured on 17 pore fluid and 13 sediment samples from Sites 1253 and 1254 drilled offshore Costa Rica during Ocean Drilling Program (ODP) Leg 205. An additional 83 pore fluid and 29 sediment samples were analyzed for Ba concentrations from Sites 1039 and 1040 drilled during ODP Leg 170 offshore Costa Rica. Sites 1039/1253 and 1040/1254 are part of a transect across the Middle America Trench offshore Nicoya Peninsula. The entire incoming sediment section is being underthrust beneath the margin, providing an ideal setting to examine Ba cycling in the shallow levels of the subduction zone. Results from these analyses indicate that a significant amount of Ba is liberated from the mineral barite (BaSO4) in the uppermost hemipelagic sediments arcward of the trench. The shallow distillation of Ba may impact the amount of sedimentary Ba reaching the deeper subduction zone.

Carbon and oxygen isotopic composition and age dating of sediments from ODP Leg 170 sites and Hole 205-1253A

In this data report we present results from stable isotope measurements (d13C and d18O) on bulk sediment at several sites located on a transect along a subduction margin offshore Costa Rica (Ocean Drilling Program Sites 1039, 1040, and 1253). Comparison of stable isotope compositions (d13C and d18O) of the pelagic carbonates Subunit U3C between the reference sites (Site 1039 and 1253) and the underthrust section (Site 1040) reveals similar d13C values and minor differences in d18O values within four specific intervals. Isotope stratigraphy was then used to further constrain the shipboard age models based on bio- and magnetostratigraphy. The resulting age models are in agreement with those derived from biostratigraphy and confirm that the sedimentation rate of the lower Subunit 3C is roughly constant on the order of 50 m/m.y. This is in contrast with the postulated very high sedimentation rates at ~12.7 Ma and lower sedimentation rates (~18 m/m.y.) in the lower part of the section between 16 and 13 Ma, as suggested by shipboard magnetostratigraphic datums.

10Be data for sediments from DSDP Leg 56 and ODP Leg 170

Sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity (the seismogenic zone) and the subduction recycling of volatiles and continentally derived materials to the Earth's mantle. Identifying and quantifying sediment accretion, essential for a complete mass balance across the margin, can be difficult. Seismic images do not define the processes by which a prism was built, and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy. This contribution reports the first use of cosmogenic 10Be depth profiles to define the origin and structural evolution of forearc sedimentary prisms. Biostratigraphy and 10Be model ages generally are in good agreement for sediments drilled at Deep Sea Drilling Project Site 434 in the Japan forearc, and support an origin by imbricate thrusting for the upper section. Forearc sediments from Ocean Drilling Program Site 1040 in Costa Rica lack good fossil or paleomagnetic age control above the decollement. Low and homogeneous 10Be concentrations show that the prism sediments are older than 3-4 Ma, and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments. Low 10Be in Costa Rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion.

Permeabilities, grain size and carbon composition of sediments from ODP Hole 170-1040C and two Leg 205 holes

Constant-pressure difference and constant-flow permeability tests were conducted on core samples from Ocean Drilling Program Legs 170 and 205 from the Costa Rica subduction zone representing pelagic carbonate and hemipelagic mud lithologies. Seven whole-round core samples from Sites 1040, 1253, and 1255 were tested for vertical permeabilities. The permeabilities of the pelagic carbonate sediments range from ~4 x 10**-16 to ~1 x 10**-15 m**2. The permeabilities of the hemipelagic mud sediments vary from ~2 x 10**-18 to ~4 x 10**-18 m**2. To further characterize the sediments, grain size, total carbon, and total inorganic carbon analyses were conducted.