The Orca Basin is a mid-slope, silled, mini-basin in the northern

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

some 300 km southwest of the

Mississippi River The Mississippi River is the List of longest rivers of the United States (by main stem), second-longest river and chief river of the second-largest Drainage system (geomorphology), drainage system in North America, second only to the Hudson B ...

mouth on the

Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...

continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

.Meckler, A. N., et al.
''Glacial to Holocene terrigenous organic matter input to sediments from Orca Basin, Gulf of Mexico,''
Earth and Planetary Science Letters 272 (2008) 251–263. It is unique amongst the mini-basins in this area, in containing a large brine pool of

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

salt

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of that of solutions used for ...

. The pool is approximately in area and up to deepPilcher, R.S. and Blumstein, R.D. 2007. Brine volume and salt dissolution rates in Orca Basin, northeast Gulf of Mexico.

AAPG Bulletin The ''AAPG Bulletin'' is a monthly peer-reviewed scientific journal covering geosciences and associated technologies relating to the energy industry. It is an official journal of the American Association of Petroleum Geologists. The current editor ...

; 91; no. 6; p. 823-833. Abstract under depth of Gulf water and is derived from

dissolution Dissolution may refer to: Arts and entertainment Books * ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * ''Dissolution'' (Sansom novel), a 2003 historical novel by C. J. Sansom Music * Dissolution, in mu ...

of underlying

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

age

Louann Salt The Louann Salt is a widespread evaporite formation that formed in the Gulf of Mexico during the Callovian in the mid Jurassic. The Louann formed in a rift as the South American and North American Plates separated, from an embayment of the Pacific ...

. With a volume of the pool results from the dissolution of about 3.62 billion tonnes of the Louann Salt bed into seawater. The basin owes its shape to ongoing

salt tectonics upright=1.7 Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This ...

and is surrounded by salt

diapir A diapir (; , ) is a type of igneous intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh–T ...

Gas hydrates Clathrate hydrates, or gas hydrates, clathrates, hydrates, etc., are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped i ...

were detected in a number of cores collected in the Orca basin during Leg 96 of the

Deep Sea Drilling Program The Deep Sea Drilling Project (DSDP) was an ocean drilling project operated from 1968 to 1983. The program was a success, as evidenced by the data and publications that have resulted from it. The data are now hosted by Texas A&M University, alth ...

(DSDP). The cores were recovered from a water depth of at Holes 618 and 618A, with first evidence of gas hydrate occurring in Hole 618. Hydrates were observed in the top section of Core 618-4 at 85 fbsf (26 mbsf) in gray mud and consisted of a few white crystals of a few millimeters in diameter. At Hole 618A, gas hydrates were observed in both Cores 618A-2 and 618-3 in the 62-121 fbsf (19-37 mbsf) range, with hydrates distributed throughout Core 618A-3. The hydrates ranged in size from a few millimeters to possibly a centimeter in diameter and were white.DSDP Leg 96 Shipboard Science Party 1986. DSDP Leg 96 Shipboard Science Party. Site 618 Initial Report. DSDP Leg 96. Washington (U.S. Govt. Printing Office): DSDP, 1986, 399-407. Based on light

δ13C In geochemistry, paleoclimatology, and paleoceanography ''δ''13C (pronounced "delta c thirteen") is an isotopic signature, a measure of the ratio of stable isotopes 13C : 12C, reported in parts per thousand (per mil, ‰). The measure is al ...

values, the origin of the hydrate gas is

biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...

. Researchers also noted that some of the hydrates appeared to occur in the sandy layers of the cores. In contrast to other gas hydrate occurrences in the Gulf of Mexico, the gas hydrate was found within a mini-basin instead of on the fractured and faulted rim of the mini-basin.Milkov and Sassen 2000. Milkov, A., and R. Sassen. "Thickness of the natural gas hydrate stability zone, Gulf of Mexico continental slope." ''

Marine and Petroleum Geology ''Marine and Petroleum Geology'' is a peer-reviewed scientific journal covering marine and petroleum geology. It was established in 1984 and is published by Elsevier. The editor-in-chief is Massimo Zecchin ( Istituto Nazionale di Oceanografia e di ...

'' 17, no. 9 (2000): 981-991. It was also noted that the depth of gas hydrate occurrence coincides with the presence of black organic and/or

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

-rich mud.

Importance of Orca Basin as a study site

The Orca Basin is important in understanding

glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...

and deglacial changes, including the history of

meltwater Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing. Meltwater ca ...

flows from the

Laurentide Ice Sheet The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square miles, including most of Canada and a large portion of the Northern United States, multiple times during the Quaternary glacial epochs, from 2.58 million year ...

, that have affected

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

and the Gulf of Mexico.Poore, R.Z., Verardo, S., Caplan, J., Pavich, K. and Quinn, T., 2011. ''Planktic Foraminiferal Relative Abundance and Trends in Gulf of Mexico Holocene Sediments.'' in Tunnell, J.W., Felder, D.L., Earle, S.A., Buster, N.A., Holmes, C.W. and Camp, D.K. eds., p. 367-379. ''Gulf of Mexico Origin, Waters, and Biota: Volume 3, Geology (Vol. 3).'' College Station, Texas, Texas A&M University Press. . The sediments that fill the Orca Basin contain an important record of the paleoenvironment and paleo-oceanology of the Louisiana continental slope south of the

Mississippi River Delta The Mississippi River Delta is the confluence of the Mississippi River with the Gulf of Mexico in Louisiana, southeastern United States. The river delta is a area of land that stretches from Vermilion Bay on the west, to the Chandeleur Isl ...

for at least the last 25,000 years.Flower, B.P., and Kennett, J.P., 1995, ''Biotic responses to temperature and salinity changes during the last deglaciation, Gulf of Mexico'', in Stanley, S.M., ed., p. 209–220. ''Effects of past global change on life'': Washington, D.C., National Academy Press, National Research Council Studies in Geophysics. .Kennett, J.P., Elmstrom, K. and Penrose, N., 1985. ''The last deglaciation in Orca Basin, Gulf of Mexico: high-resolution planktonic foraminiferal changes.'' Palaeogeography, Palaeoclimatology, Palaeoecology, 50(2-3), pp.189-216. Because of location of this basin, paleoenvironmental proxies, e.g.

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...

ic foraminifers,

stable isotope ratio The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abunda ...

s, changes in sediment texture, and reworked calcareous nanofossils, preserved in its sediment also recorded the impact and chronology of meltwater floods that flowed down the Mississippi River on the Gulf of Mexico during the last

deglaciation Deglaciation is the transition from full glacial conditions during ice ages, to warm interglacials, characterized by global warming and sea level rise due to change in continental ice volume. Thus, it refers to the retreat of a glacier, an ice shee ...

.Brown, P., Kennett, J.P. and Ingram, B.L., 1999. ''Marine evidence for episodic Holocene megafloods in North America and the northern Gulf of Mexico.'' Paleoceanography, 14(4), pp.498-510.Marchitto, T.M. and Wei, K.Y., 1995. ''History of Laurentide meltwater flow to the Gulf of Mexico during the last deglaciation, as revealed by reworked calcareous nannofossils.'' Geology, 23(9), pp.779-782. In addition, the only recorded recovery of gas hydrates in the Gulf of Mexico from depths greater than 66 fbsf (20 mbsf) occurred at DSDP Site 618 in the Orca Basin.Hutchinson, D.R., Ruppel, C.D., Roberts, H.H., Carney, R.S. and Smith, M., 2011. ''Gas hydrates in the Gulf of Mexico.'' in Tunnell, J.W., Felder, D.L., Earle, S.A., Buster, N.A., Holmes, C.W. and Camp, D.K. eds., p. 247-276. ''Gulf of Mexico Origin, Waters, and Biota: Volume 3, Geology (Vol. 3).'' College Station, Texas, Texas A&M University Press. . The recovery of biogenic

methane hydrate Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...

from Orca basin is also significant due to the high

salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...

values, which at the sediment/water interface were nearly five times as high as those found in the

Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...

(with salinity values of 240-260 PSU). The values decreased rapidly with depth to about 98 fbsf (30 mbsf) before becoming constant (48-56 PSU). The hydrate recovered from both sites in Orca Basin were in the range of 85-121 fbsf (26-37 mbsf) and are physical evidence of the decreased salinity levels. The Orca Basin provides an ideal setting for studying the fate of organic matter, nutrients, and metals. Examining the consumption or production of dissolved materials provides insight to how said materials mix with the seawater. At depths of to the distribution of ammonium reflects conservative mixing of the ammonium with seawater. At a depth of , denitrification is heavily limited due to the absence of nitrate. With the absence of nitrate, manganese and iron oxides are more present which also leads to iron-reducing and manganese reducing bacteria being more present. The changing presence of materials at different depth of Orca Basin signifies what heterotrophic populations are present. Below depths of , detectible dissolve sulfide increases in presence and indicates that bacterial sulfate reduction is the primary method for organic matter degradation.

References

External links

Carney, Bob, ''Lakes Within Oceans,'' Ocean Explorer

{{Coord, 26, 56, 46, N, 91, 20, 44, W, display=title Anoxic waters Gulf of Mexico Sedimentary basins of North America