Warm Saline Bottom Water Research Articles

Phanerozoic Large Igneous Provinces (LIPs), HEATT (Haline Euxinic Acidic Thermal Transgression) episodes, and mass extinctions

The Phanerozoic consists of three mutually exclusive climate states: Icehouse, Greenhouse, and Hothouse. The Greenhouse is the default state, representing > 70% of the Phanerozoic. It is characterized by a weak thermal mode (pole-driven deep-water formation), with little or no continental sea ice, and weakly oxic cool-water sinking near the poles. Nitrogen limitation characterizes Greenhouse oceans in which ammonium may be the dominant bioavailable form of nitrogen. The steady-state Greenhouse default must be forced to produce either of the other two states. Continent–continent collision produces the intense chemical weathering that draws down atmospheric CO 2 and results in the Icehouse state that comprises 20–25% of the Phanerozoic. Icehouses have ice caps in contact with the ocean that produce icebergs and foster sea-ice formation. Sea-ice formation fills the ocean with highly oxic phosphorus-limited icewater except for a thin layer of warm surface water in the tropics. Of the 23 most conspicuous mass extinctions of the Phanerozoic (including the late Ediacaran), 4 are associated with ice-volume-related regressions, and 3 are problematic. We suggest that the other 16 mass extinctions occurred when Greenhouse climate was forced to Hothouse climate. Large Igneous Provinces (LIPs) can trigger the HEATT (Haline Euxinic Acidic Thermal Transgression) mechanism, which can force Greenhouse climate to a Hothouse climate state. The rapid, voluminous CO 2 emissions of LIPs cause a series of events that result in very slow but pronounced haline-mode circulation (mid-latitude-driven deep-water formation) that produces warm saline bottom water at very dry, hot subtropical and middle latitudes. This water upwells at the poles, causing warm and moist equable climate. Hothouse development promotes deep-sea anoxia that is generated in concert with CO 2 buildup and warming. The spread of anoxia favors reduced nitrogen over nitrate, and limited iron dust curtails nitrogen fixation. This leads to dominance of primary productivity by primitive green algae and/or cyanobacteria. Progression to euxinia that reaches the photic zone severely curtails primary productivity via poisoning, even as availability of usable nitrogen plummets. The result is eukaryotic mass extinction. We suggest that the 16 Phanerozoic mass extinctions referred to above resulted at least in part from the HEATT process. Evidence for this relationship at 11 of the most severe extinctions is convincing. Data sets for the other 5 are strongly suggestive of HEATT-related mass extinction, but do not yet have enough documentation to reliably characterize them as HEATT episodes.

An 80 million year oceanic redox history from Permian to Jurassic pelagic sediments of the Mino-Tamba terrane, SW Japan, and the origin of four mass extinctions

Fabric and pyrite framboid size analysis of Permian to Jurassic samples from the Mino-Tamba terrane of Japan provide an 80 myr redox history from the Panthalassa Ocean. Fully oxygenated conditions dominated but were punctuated by three phases of anoxia (euxinia) during the Permo-Triassic boundary, the late Early Triassic (Spathian Stage) and the late Early Jurassic (Toarcian Stage). A Permo-Triassic superanoxic event of 10 myr duration is thus resolved into a complex redox history of dysoxic-anoxic background conditions punctuated by short-lived euxinic pulses. Both the end-Permian and Toarcian anoxic episodes began abruptly with the development of siliceous claystones, characterized by a framboid-dominated pyrite fraction and no organic C enrichment, interpreted to record euxinic conditions. These facies are overlain by organic-rich shales with slightly larger framboids, more crystalline pyrite and bioturbation indicating accumulation beneath a better-ventilated, higher productivity water column. Thus, an initial phase of ocean-wide stagnation was followed by improved circulation attributed to the onset of warm saline bottom water circulation. Five radiolarian crises during our study interval show a complex relationship with redox conditions. The latest Permian mass extinction coincided with the onset of ocean euxinia but two other crises (Middle Permian and end Triassic), occurred during fully oxygenated intervals. Radiolarian radiation events are also not consistently linked with redox conditions: diversification in the early Late Permian coincides with the onset of weakly dysoxic conditions, late Early Triassic and late Early Jurassic radiations coincide with euxinic events, and the basal Jurassic radiation occurred during a fully oxic phase.

Causes and consequences of extreme Permo-Triassic warming to globally equable climate and relation to the Permo-Triassic extinction and recovery

Permian waning of the low-latitude Alleghenian/Variscan/Hercynian orogenesis led to a long collisional orogeny gap that cut down the availability of chemically weatherable fresh silicate rock resulting in a high-CO 2 atmosphere and global warming. The correspondingly reduced delivery of nutrients to the biosphere caused further increases in CO 2 and warming. Melting of polar ice curtailed sinking of O 2- and nutrient-rich cold brines while pole-to-equator thermal gradients weakened. Wind shear and associated wind-driven upwelling lessened, further diminishing productivity and carbon burial. As the Earth warmed, dry climates expanded to mid-latitudes, causing latitudinal expansion of the Ferrel circulation cell at the expense of the polar cell. Increased coastal evaporation generated O 2- and nutrient-deficient warm saline bottom water (WSBW) and delivered it to a weakly circulating deep ocean. Warm, deep currents delivered ever more heat to high latitudes until polar sinking of cold water was replaced by upwelling WSBW. With the loss of polar sinking, the ocean was rapidly filled with WSBW that became increasingly anoxic and finally euxinic by the end of the Permian. Rapid incursion of WSBW could have produced ∼20 m of thermal expansion of the oceans, generating the well-documented marine transgression that flooded embayments in dry, hot Pangaean mid-latitudes. The flooding further increased WSBW production and anoxia, and brought that anoxic water onto the shelves. Release of CO 2 from the Siberian traps and methane from clathrates below the warming ocean bottom sharply enhanced the already strong greenhouse. Increasingly frequent and powerful cyclonic storms mined upwelling high-latitude heat and released it to the atmosphere. That heat, trapped by overlying clouds of its own making, suggests complete breakdown of the dry polar cell. Resulting rapid and intense polar warming caused or contributed to extinction of the remaining latest Permian coal forests that could not migrate any farther poleward because of light limitations. Loss of water stored by the forests led to aquifer drainage, adding another ∼5 m to the transgression. Non-peat-forming vegetation survived at the newly moist poles. Climate feedback from the coal-forest extinction further intensified warmth, contributing to delayed biotic recovery that generally did not begin until mid-Triassic, but appears to have resumed first at high latitudes late in the Early Triassic. Current quantitative models fail to generate high-latitude warmth and so do not produce the chain of events we outline in this paper. Future quantitative modeling addressing factors such as polar cloudiness, increased poleward heat transport by deep water and its upwelling by cyclonic storms, and sustainable mid-latitude sinking of warm brines to promote anoxia, warming, and thermal expansion of deep water may more closely simulate conditions indicated by geological and paleontological data.

“Warm saline bottom water” during the LPTM?

“Warm saline bottom water” during the LPTM?

Continental runoff and early Cenozoic bottom-water sources

The dominance of warm saline bottom water during the mid-Cretaceous and the early Cenozoic has been inferred from sea-floor sediment records, an interpretation supported by early ocean general circulation model experiments. Thermohaline circulation depends in part on upper ocean salinities; however, early ocean models neglected continental runoff, a potentially critical factor in the salinity budget of the surface ocean. Our early Eocene ocean model sensitivity tests show that model deep-water sources can be enhanced, diminished, or turned off by varying the treatment of continental runoff in the atmosphere-ocean moisture flux calculation. Failure to treat surface runoff adequately thus has important implications for the simulation of thermohaline flow and formation of warm saline bottom water. Variations in runoff could have led to rapid changes in the relative importance of high-latitude versus subtropical deep water, such as may have occurred during the late Paleocene–early Eocene boundary interval (≈ 53.6–56.2 Ma).

The Late Paleocene (Thanetian) Hydrological Cycle and Implications for the Creation of Warm-Saline Bottom Water: GISS GCM Simulations: ABSTRACT

The Late Paleocene (Thanetian) Hydrological Cycle and Implications for the Creation of Warm-Saline Bottom Water: GISS GCM Simulations: ABSTRACT

Past CO2 Changes and Tropical Sea Surface Temperatures

The relationship between past CO2 variations and tropical climate change on time scales of millions of years is reviewed. Explanations for Cretaceous and early Cenozoic climate change often involve higher atmospheric CO2 levels. Climate models predict that such increases should have been associated with warmer tropical sea surface temperatures (SSTs). However, sparse mid‐Cretaceous δ18O‐based SST estimates from low‐latitude calcareous plankton are ∼ 1.0–1.5‰ heavier than predicted. Initial evaluations indicate that changes in salinity may not alleviate model‐data discrepancies for calcareous δ18O and that interpretation of altered latitudinal δ18O gradients in terms of increased poleward heat transport due to warm, saline bottom water (WSBW) formation would require very large levels of WSBW production (at least 60 Sv). Calcareous δ18O records may not always record true SST patterns, as δ18O analyses from phosphatic groups, which are less sensitive to diagenesis, indicate SSTs ∼5°C higher. This latter conclusion is supported by a number of other fossil groups, which indicate temperatures warmer than those estimated from calcareous δ18O. In order to more rigorously test the paleo‐CO2 hypothesis, and indeed the models used to forecast future climates, it is desirable to intensify investigations of past climate change in low latitudes in order to determine whether these preliminary conclusions stand the test of further scrutiny.

Manganese deposition through geological time: dominance of the post-eocene deep-sea environment

The total terrestrial manganese ore deposits amount to about 6.4×10 9 tonnes of which 5.3×10 9 tonnes (82%) are Precambrian and 1.1×10 9 tonnes (18%) Mesozoic and Cenozoic. Deep-sea manganese nodules formed since the lower Miocene unconformity (12 Ma B.P.) hold about 10 11 tonnes Mn (about 16 times the manganese in terrestrial deposits). The development of deep-sea manganese nodules is a reflection of global cooling and resulted from the transition of the sluggish Cretaceous ocean with warm saline bottom water to the post-Eocene ocean with cold, well-oxygenated bottom curents. Important climatic and oceanographic events occurred at the Eocene-Oligocene boundary and more particularly at the lower Miocene unconformity which marks the period of increased Antarctic Bottom Water circulation. By contrast, the giant shallow-water manganese deposits of the Lower Jurassic to Oligocene are associated with anoxia and high sea-level stands. The two types of deposit reflect different oceanographic conditions and tend not to be contemporaneous. Mass balance considerations suggest that the present-day input of manganese into the oceans is in balance with its deposition in manganese nodules and sediments with perhaps 90% of the input being from hydrothermal sources, although this is speculative. The scale of present-day deep-sea manganese nodule formation suggests that we live in the manganese era.

Extinction, survivorship and evolution of planktic foraminifera across the Cretaceous/Tertiary boundary at El Kef, Tunisia

An expanded sediment record at El Kef shows that the K/T boundary extinctions of planktic foraminifera extend over an interval from 25 cm below the geochemical boundary (Ir anomaly) to 7 cm above. Species extinctions appear sequential with complex, large, ornate forms disappearing first and smaller, less ornate, forms surviving longer. The 14 species extinctions below the boundary appear unrelated to an impact event. Cretaceous species survivorship is greater than previously assumed. About 10 species survive (22%) into Subzone P1a ( Globigerina eugubina). All Cretaceous survivors are small primitive forms which are generally smaller than their ancestors in Cretaceous sediments. Species evolution after the K/T event occurs in two pulses. The first new Paleocene species evolve in the basal black clay (Zone PO) immediately after the major Cretaceous extinctions. Evolving species are small and primitive similar to Cretaceous survivors. The second pulse in species evolution occurs in the lower part of Subzone P1b with the appearance of larger more diverse species. The first major increase in carbonate sedimentation and productivity occurs at this time and signals the recoveyr of the ecosystem nearly 300,000 years after the K/T event. The species extinctions prior to the generally assumed impact event implied by the Ir anomaly, and the long recovery period of the ecosystem thereafter cannot be explained by a single impact, but suggest that multiple causes may be responsible such as climatic changes, volcanism, a sea level drop, production of warm saline bottom water and the chemical consequences associated with increased salinity.

Cenozoic ice-volume history: Development of a composite oxygen isotope record

To view Cenozoic ice-volume and ocean-temperature history, a composite δ 18 O time series was constructed for tropical shallow-dwelling planktonic foraminifers and for deep-water benthic foraminifers. We conclude that Cenozoic benthic δ 18 O primarily reflect considerable deep-ocean temperature variation and that tropical planktonic δ 18 O afford the best record of Cenozoic ice volume. The planktonic composite indicates the presence of a significant ice budget for the past 40 m.y. The difference between the benthic and planktonic δ 18 O composites suggests that low-latitude production of warm saline bottom water dominated Tertiary thermohaline circulation.

Stratification and paleocirculation of the Late Cretaceous Western Interior Seaway of North America

Paleotemperature and salinity profiles for the Western Interior Seaway are reconstructed by use of oxygen and carbon isotopic signatures of shell material. Data have been gathered on different groups of molluscs distributed in three distinct habitat types: infaunal benthonic, epifaunal benthonic, and nektonic. Isotopic variations and inferred environmental tolerances of the organisms were used to determine upper and lower temperature boundaries for each habitat group and therefore water-depth zones. Water density for each temperature-salinity combination within a habitat range was calculated and used to develop a physically stable stratification model. The results clearly show that the water column consisted of denser, more saline, warm bottom layers that were overlain by, and decoupled from, cooler, less saline intermediate layers. It is proposed that this profile originated due to a surface current transporting runoff water from the western highlands toward the eastern shallow border of the seaway where evaporation and heating of surface water in a warm, humid atmosphere caused increased density and sinking, followed by westward return flow of the warm saline bottom water. This model is compatible with known climatic and oceanographic information for the Cretaceous.

Oxygen isotope ranking of late Eocene and Oligocene planktonic foraminifers: Implications for Oligocene sea-surface temperatures and global ice-volume

Oxygen isotope analyses of late Eocene and Oligocene planktonic foraminifers from low and middle latitude sites in the Atlantic Basin show that different species from the same samples can yield significantly different isotopic values. The range of isotopic values observed between species is greatest at low-latitudes and declines poleward. Many planktonic foraminifers exhibit a systematic isotopic ranking with respect to each other and can therefore be grouped on the basis of their isotopic ranking. The isotopic ranking of some taxa, however, appears to vary geographically and/or through time. Isotopic and paleontologic data from DSDP Site 522 indicate that commonly used isotopic temperature scales underestimate Oligocene sea surface temperatures. We suggest these temperature scales require revision to reflect the presence of Oligocene glaciation. Comparison of isotopic and paleontologic data from Sites 522, 511 and 277 suggests cold, low-salinity surface waters were present in high southern latitudes during the early Oligocene. Lowsalinity, high latitude surface waters could be caused by Eocene/Oligocene paleogeography or by the production of warm saline bottom water.

Warm saline bottom water in the ancient ocean

Considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past. A simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water. It is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water. Evidence from the palaeotemperature record and palaeogeography during the late Cretaceous is used to reinforce the hypothesis. Climatic and chemical consequences of the formation of warm saline bottom water are discussed.