Sedimentary record of Holocene/recent environmental changes, Ordy Pond, Oahu, Hawaii [abs.]

TitleSedimentary record of Holocene/recent environmental changes, Ordy Pond, Oahu, Hawaii [abs.]
Publication TypeJournal Article
Year of Publication1999
AuthorsTribble, JS, Garrison GH, Grabowski EM, Glenn CR
Volume80
Issue46
PaginationF486
Abstract

Ordy Pond occupies a sinkhole in the Pleistocene caprock of Oahu, Hawaii. Pond sediments cored to a depth of 17.5 m have a bottom age of 25,700 yr, and contain a continuous record of sedimentation since 10,000 yr BP. These sediments are an invaluable archive of environmental, climatic, and sea level changes on Oahu during the late Pleistocene/Holocene. Three stratigraphic units are identified. The bottom-most unit is 4 m of gray shell hash containing terrigenous snails and pebbles of reef rock in a fine-grained carbonate matrix. This unit grades up into a thin peat-like interval, marking the terrestrial-aquatic transition. The base of the transition is dated at 9780 yr BP. The middle unit comprises 8.5 m of finely laminated to thinly bedded authigenic carbonate and organic matter, with interbeds of diatom ooze. The upper-most 5 m of core are organic-rich, gelatinous to spongy, sediment. Faint color banding is present throughout the upper unit. Diatoms are the most abundant fauna, and are dispersed throughout the middle and upper units. A sharp boundary, dated at 150 yr BP by first appearance of historic pollen, separates the dark, relatively uniform sediment of the upper unit from the underlying laminated sediment. Striking differences in appearance of the middle and upper units are associated with compositional differences. The middle, laminated, unit is more carbonate-rich (average of 56%), has a slightly lower organic matter content (20%), and is much more variable in composition than the upper unit (45% carbonate and 22% organic matter). Calcite is the dominant carbonate phase in the middle unit, whereas aragonite dominates the upper unit. Isotopic compositions of the carbonates of the two units also differ. Middle unit carbonates have dominantly negative d13C values (+1.6 to -8.1 °/oo) and positive d18O values (-0.1 to +3.5 °/oo). In contrast, d13C values of the upper unit carbonates are al positive (0.2 to 7.5 °/oo) and d18O values vary from -1.4 to +5.4 °/oo. Carbon and oxygen isotopes vary inversely in the middle unit, but covary in the upper unit. Differences in sediment composition and character between the two units reflect factors including vegetation and land use changes, introduction of nonnative species, and lowering of the water table due to pumping for crop irrigation. Water column chemistry was studied to delineate the environment of deposition of upper unit sediments. Maximum water depth is 5 m. Salinity is 21-22 °/oo and there is no salinity or temperature stratification. Oxygen concentrations are low at the surface (1.6 mg/L) and drop to zero at a depth of 4.3 m. Sulfate reduction occurs throughout the water column. High levels of primary productivity are indicated by chlorophyll-A concentrations in excess of 90 mg/kg. Nutrient concentrations are high (NH4+ up to 286 mM and PO43- up to 8.7 mM). A subsurface peak in NO3- indicates active nitrification at a depth of 0.5-1 m. High NIP ratios of near-surface waters suggest nitrogen fixation may be important. High alkalinities (~21.5 meq/kg) result in high degrees of carbonate supersaturation. Local minima in dissolved Ca2+ and alkalinity are consistent with authigenic precipitation of carbonate at a water depth of ~1m. Although dissolved carbon isotopic values are negative (-3 to -4 °/oo), isotopic fractionation during aragonite precipitation accounts for the slightly positive d13c values characteristic of surface sediment carbonates.

URLhttps://irma.nps.gov/App/Reference/Profile/641704
Original PublicationEos, Transactions, American Geophysical Union, supplement
Source OrganizationNational Park Service
Source ProjectIRMA portal