Geology topics

F.T. MacKenzie

Publications and source records attributed to F.T. MacKenzie.

7 recordsLinked to original sources

USGS2009

Net Loss of CaCO3 from a subtropical calcifying community due to seawater acidification: Mesocosm-scale experimental evidence

Acidification of seawater owing to oceanic uptake of atmospheric CO 2 originating from human activities such as burning of fossil fuels and land-use changes has raised serious concerns regarding its adverse effects on corals and calcifying communities. Here we demonstrate a net loss of calcium carbonate (CaCO 3 ) material as a result of decreased calcification and increased carbonate dissolution from replicated subtropical coral reef communities (N=3) incubated in continuous-flow mesocosms subject to future seawater conditions. The calcifying community was dominated by the coral Montipora capitata. Daily average community calcification or Net Ecosystem Calcification (NECC=CaCO 3 production - dissolution) was positive at 3.3 mmol CaCO 3 m -2 h -1 under ambient seawater pCO 2 conditions as opposed to negative at -0.04 mmol CaCO 3 m -2 h -1 under seawater conditions of double the ambient pCO 2 . These experimental results provide support for the conclusion that some net calcifying communities could become subject to net dissolution in response to anthropogenic ocean acidification within this century. Nevertheless, individual corals remained healthy, actively calcified (albeit slower than at present rates), and deposited significant amounts of CaCO 3 under the prevailing experimental seawater conditions of elevated pCO 2 .

Biogeosciences↗

USGS2008

Ocean acidification and calcifying reef organisms: A mesocosm investigation

A long-term (10 months) controlled experiment was conducted to test the impact of increased partial pressure of carbon dioxide (pCO2) on common calcifying coral reef organisms. The experiment was conducted in replicate continuous flow coral reef mesocosms flushed with unfiltered sea water from Kaneohe Bay, Oahu, Hawaii. Mesocosms were located in full sunlight and experienced diurnal and seasonal fluctuations in temperature and sea water chemistry characteristic of the adjacent reef flat. Treatment mesocosms were manipulated to simulate an increase in pCO2 to levels expected in this century [midday pCO2 levels exceeding control mesocosms by 365 ?? 130 ??atm (mean ?? sd)]. Acidification had a profound impact on the development and growth of crustose coralline algae (CCA) populations. During the experiment, CCA developed 25% cover in the control mesocosms and only 4% in the acidified mesocosms, representing an 86% relative reduction. Free-living associations of CCA known as rhodoliths living in the control mesocosms grew at a rate of 0.6 g buoyant weight year-1 while those in the acidified experimental treatment decreased in weight at a rate of 0.9 g buoyant weight year-1, representing a 250% difference. CCA play an important role in the growth and stabilization of carbonate reefs, so future changes of this magnitude could greatly impact coral reefs throughout the world. Coral calcification decreased between 15% and 20% under acidified conditions. Linear extension decreased by 14% under acidified conditions in one experiment. Larvae of the coral Pocillopora damicornis were able to recruit under the acidified conditions. In addition, there was no significant difference in production of gametes by the coral Montipora capitata after 6 months of exposure to the treatments. ?? 2008 Springer-Verlag.

Coral Reefs↗

USGS1989

Geohydrology of the Laura fresh-water lens, Majuro atoll: A hydrogeochemical approach

In small limestone islands, the depositional history and subsequent chemical interactions between ground water and the aquifer host rock play critical roles in the occurrence, movement, and chemical quality of ground water. The hydrogeochemistry of the Laura fresh-water lens, Majuro atoll, Marshall Islands, is an example of these relations. Laura is underlain by two principal hydrologic units. The upper unit is a back-reef-marginal-lagoonal deposit which formed during the Holocene interglacial stage. It is composed of moderately permeable carbonate sediments. The lower hydrologic unit consists of highly permeable limestone that was subaerially exposed, most likely during a Pleistocene glacial lowstand. Similar stratification is found at Bikini and Enewetak atolls. The upper hydrologic unit contains a calcium bicarbonate-rich fresh-water lens, in which a potable fresh-water nucleus as much as 14 m thick occurs on the lagoon side of the island. Storage in the fresh-water nucleus ranged from 1.70 x 10 6 to 2.08 x 10 6 m 3 during 1984-1985. Ground-water occurrence and flow are governed by an asymmetric distribution of lithofacies about the longitudinal axis of the island and an abrupt increase in permeability at the contact between the upper and lower hydrologic units. The highly permeable lower hydrologic unit contains sea water and truncates the fresh-water-sea-water mixing zone. The fresh-water lens and associated fresh-water-sea-water mixing zone are the site of continuously occurring diagenetic reactions that significantly affect the porosity and permeability of the aquifer. Non-equilibrium dissolution-precipitation reactions, coupled with variations in CO 2 input, control the chemical evolution of Laura ground water. At the present rate of chemical weathering, 465 m 3 of sediment are being dissolved and transported to the sea by ground water each year. This dissolution results in an annual increase in porosity of 0.01%. The primary factors controlling the occurrence and flow of ground water in the leeward reef islet of Laura are (1) the depositional history of the upper hydrologic unit, which has resulted in a greater accumulation of low-permeability (fine-grained) sediments beneath the lagoon side of the island and a high- to low-permeability (coarse-to fine-grained sediment) gradation between the ocean and lagoon; and (2) the diagenetic history of the lower hydrologic unit, which has resulted in a highly permeable basement.

Geological Society of America Bulletin↗

USGS1982

Organic geochemistry and pore water chemistry of sediments from Mangrove Lake, Bermuda

Mangrove Lake, Bermuda, is a small coastal, brackish-water lake that has accumulated 14 m of banded, gelatinous, sapropelic sediments in less than 10 4 yr. Stratigraphic evidence indicates that Mangrove Lake's sedimentary environment has undergone three major depositional changes (peat, freshwater gel, brackish-water gel) as a result of sea level changes. The deposits were examined geochemically in an effort to delineate sedimentological and diagenetic changes. Gas and pore water studies include measurements of sulfides, ammonia, methane, nitrogen gas, calcium, magnesium, chloride, alkalinity, and pH. Results indicate that sulfate reduction is complete, and some evidence is presented for bacterial denitrification and metal sulfide precipitation. The organic-rich sapropel is predominantly algal in origin, composed mostly of carbohydrates and insoluble macromolecular organic matter called humin with minor amounts of proteins, lipids, and humic acids. Carbohydrates and proteins undergo hydrolysis with depth in the marine sapropel but tend to be preserved in the freshwater sapropel. The humin, which has a predominantly aliphatic structure, increases linearly with depth and composes the greatest fraction of the organic matter. Humic acids are minor components and are more like polysaccharides than typical marine humic acids. Fatty acid distributions reveal that the lipids are of an algal and/or terrestrial plant source. Normal alkanes with a total concentration of 75 ppm exhibit two distribution maxima. One is centered about n -C 22 with no odd/even predominance, suggestive of a degraded algal source. The other is centered at n -C 31 with a distinct odd/even predominance indicative of a vascular plant origin. Stratigraphic changes in the sediment correlate to observed changes in the gas and pore water chemistry and the organic geochemistry.

Mangrove Lake↗

USGS1976

Hydrogeochemistry of Bermuda: A case history of ground-water diagenesis of biocalcarenites

Bermuda is composed of relatively young skeletal limestones currently undergoing diagenesis by the ground water passing through them. The saturated zone consists of separate fresh-water bodies laterally surrounded and underlain by extensive brackish aureoles, in which the meteoric water is mixed with sea water. The meteoric water enters the aquifer after passing through the soil or through marshes (outcrops of the ground-water bodies), in each case causing an influx of CO 2 to the saturated zone. Examination of the ground-water chemistry enables mapping of (a) the extent of mixing of meteoric ground water and sea water; (b) P CO2 ; (c) the extent of saturation with calcite and aragonite; (d) concentration of Sr; and (e) the amount of calcium and magnesium derived from the limestones. It is concluded that three processes control the chemistry of Bermudian ground water: (a) generation of elevated CO 2 partial pressures in soils and marshes; (b) dissolution of metastable carbonate minerals (principally aragonite); and (c) mixing with sea water. Bermuda ground water apparently approaches a steady state of aragonite dissolution (at slight subsaturation) and concurrent precipitation of calcite cement. Large Sr/Ca ratios in the ground water indicate that the dissolution of aragonite is incongruent. Dissolution is most pronounced near the marshes where CO 2 content is highest. Mixing with sea water is not significant in controlling calcite saturation. Only small amounts of magnesium enter the ground water by incongruent dissolution of magnesium calcite, an apparently slow process on the time scale of passage of the ground water through the saturated zone. All of the waters are well undersaturated with respect to dolomite. It is estimated that the present rate of recrystallization of aragonite to calcite is about 0.32 cm 3 of aragonite to calcite per m 3 of the saturated zone per year. At the present rate of chemical weathering, 360 m 3 of the saturated zone is lost each year through solution and transported to the sea by ground water.

Geological Society of America Bulletin↗

USGS1975

Rates of dissolution of aluminosilicates in seawater

Dissolution of eight clay minerals, four zeolites, and quartz in seawater has been monitored for 8 1 2 years. For most of the minerals, dissolution can be described as a first-order reaction in which dissolved silica approaches from undersaturation steady concentration values with time. Characteristic reaction rate constants (k 1 ) are of the order of 10 -7 sec -1 . One of the zeolites, clinoptilolite, shows a different dissolution behavior: SiO 2 concentration in solution reaches a high value within one year, followed by a decline to a lower value, suggestive of precipitation of another silicate phase (possibly sepiolite). A mathematical solution is given for a kinetic equation combining the parabolic-rate and first-order rate processes. It is shown that in a wide range of silicate dissolution reactions taking place over long periods of time, the presence of the parabolic-rate dissolution processes cannot be detected, thereby making its inclusion in the kinetic equations unnecessary. The experimental rates of dissolution are comparable to the SiO 2 - dissolution rates in oceanic sediments near the sediment/water interface. But deeper in the sediment, the calculated dissolution rates are significantly lower than the near-interface and experimental values. © 1975.

Earth and Planetary Science Letters↗

USGS1969

Temporal variation of alkaline earth element/chlorinity ratios in the sargasso sea

An open ocean hydrographic station located 14 miles SE of the Bermuda Islands was sampled at two week intervals through a vertical profile of 2600 meters and over the period June 1966 to March 1967. 428 samples were analyzed for Ca, Mg, Sr and chlorinity. Large temporal variations in element/chlorinity ratios were observed throughout the water column.

Earth and Planetary Science Letters↗