Tectonic activity is one of many key factors affecting Thermohaline Ocean Circulation (THOC). Continental configuration makes up the outer boundaries to the flow of seawater around the Earth, and a slight change or modification in these boundaries can have a lasting effect on these global circulation patterns. However, plate rearrangement is just a part of the mechanisms in control. It has been suggested that hydrothermal venting, seafloor rifting, and volcanic eruptions can also impact the way the density driven circulation is perpetuated. By fundamentally changing the characteristics of seawater (temperature and density) these processes slowly weaken, or in some cases intensify, deep and equatorial water production. The timescale of change has been found to be anywhere from tens of thousands of years to millions depending on the speed and magnitude of such events. Changes in THOC have been discovered within the geologic record of the Paleocene-Eocene boundary, mid-Cretaceous, and late Neogene. Understanding how these processes can alter THOC can help in the reconstruction of paleoclimate and may offer a new view on the constraints of THOC. Here, I compile a review of works, which suggest that tectonic activity plays a larger role in the adjustment of THOC than what is normally considered.
Tectonic activity plays an important role in shaping terrestrial and marine environments. These geologic events can create, destroy, and shape Earth’s surfaces within minutes or over the course of millions of years. While there is a fairly sound understanding of these processes on land, we have much to learn about what role these forces play beneath the ocean. Here, I will discuss the implications of tectonically driven sea-floor morphology, hydrothermal venting, and plate adjustment. Then I will review how these processes can affect ocean circulation, on what time scale these processes work, and where on Earth this is occurring or has occurred in the past.
Ocean circulation is constrained by continental configuration (Rea et al., 1990). Continents make up the outer boundaries of circulation; therefore, we can determine that a change in these continental boundaries would mark a change in how these currents are driven. Thus an opening (or closing) will affect global transport pathways of heat, moisture, oceanic and atmospheric chemistry and CO2, as volcanism and hydrothermal activity increase during continental and oceanic seafloor rifting (Rea et al., 1990). Globally, these currents work together. When a change is introduced to this system it responds accordingly so that oceanic heat exchange and mixing can move towards equilibrium. In the following, I assert that global ocean currents are greatly affected by tectonic (rifting, volcanism, and hydrothermal) activity leading to fundamental changes in density driven circulation.
Thermohaline Overturning Circulation and Tectonics
To understand how ocean circulation may be altered, a basic understanding of the mechanisms that move such vast volumes of water is necessary. While the world’s oceans move primarily due to internal mixing driven by mass modification, it is helpful to keep in mind that the mixing is also aided by winds which induce upwelling and surface mixing (Schmitz et al., 1998). Formally, thermohaline-overturning circulation [THOC] (Figure 1.) is the conveyor-like density driven system controlled by fluxes of heat and fresh water across the surface and subsequent interior mixing of heat and salt (Broeker, 1997; Rahmstorf, 2006).
Figure 1. A simplified illustration of global THOC where red lines indicate warm surface waters heated mostly at tropical and mid-latitudes, while blue lines indicate the colder, more dense deep water. Deep-water formation is marked in high latitudes around Greenland and Antarctica.
High latitude cooling is the primary driver of THOC. Within these regions, cold temperatures cool the saline ocean waters making it much more dense than the surrounding surface waters (Schmitz, 1998). Estimates put the displacement of Antarctic bottom water at 10 to 15×106 m3/s (Gordon, 2006). To put this into perspective, the average ocean temperature is 3.5OC, and roughly 75% of ocean water is cooler than 4O C, with only 10% of water volume being greater than 10O C (Gordon, 2006). Increasing seawater density creates a zone of downwelling where deep water is formed. As these waters sink they cause upwelling in other areas (a process that brings cool water towards the surface) (Rahmstorf, 2006). Rahmstorf (2006) explains that if high latitude cooling and surface warming were the only mechanisms of control, this system would reach equilibrium, as the temperature gradient reduces, and come to a halt. Thus, Sandström’s theorem suggested that the vertical mixing of heat at a greater depth than cooling was required to keep the currents fighting for equilibrium, therefore driving the conveyor belt of the ocean (Rahmstorf, 2006).
Closing of the Isthmus of Panama
A classic example of tectonic interference with ocean circulation is the closing of the Isthmus of Panama during the late Neogene (Schneider and Schmittner, 2006).
Haug and Tiedemann (1998) suggest that the closing of the Isthmus of Panama from 13 to 1.9Ma instigated a change in Atlantic and Pacific circulation. Coincidentally, Schneider and Schmittner (2006) posit that this gradual closing lead to an intensification of north Atlantic THOC. Sediment records from Caribbean, Atlantic, and Pacific deep sea drilling programs allowed researchers to narrow the beginning of circulation change, ranging from approximately 4.4 to 4.6Ma (Haug and Tiedemann, 1998; Schneider and Schmittner, 2006). Relatively speaking, this means that a change in circulation at this time took roughly 2.5-2.7Ma (4.4; 4.6Ma – 1.9Ma = 2.5; 2.7Ma) to initiate and fully develop. The continental emergence is said to have strengthened the Gulf Stream bringing warm saline waters to high northern latitudes, and strengthened deep-water formation in the Labrador Sea (Haug and Tiedemann, 1998). Consistent with sediment records from Bjorn Drift (just south of Iceland), and improving evidence, sea surface temperatures increased 2-3OC around 2.8Mya due to shoaling, just prior to complete closing of the gateway (Schneider and Schmittner, 2006).
More importantly the closing of the Panama gateway resulted in the establishment of the modern global circulation (Schneider and Schmittner, 2006). This underlies the profound effect tectonics have on shaping modern climate, atmospheric conditions, and THOC. It almost seems intuitive that a closing pathway would change circulation, such as the case when a valve is closed on a pipe or when a river channel diverts it’s flow to circumnavigate an obstacle. However, what other tectonic processes can facilitate this change? A look into studies conducted on the Paleocene-Eocene boundary and mid-Cretaceous may provide an answer to this question.
The Paleocene-Eocene boundary has been a source of interest for geologist, paleoclimatologists, geochemists, and biologists alike for years. At the end of the Paleocene, in a relatively short period of time (~6-30Ka), rapid oxygen and carbon isotope shifts reflected oceanographic changes and global warming that caused considerable deep-sea benthic extinctions and ocean acidification (Kennett and Stott, 1991; Zachos et al., 2005). Kennett and Stott (1991) suggest that during this time ocean circulation underwent a fundamental change. Global sea–surface temperatures increased, ranging from 5oC in mid latitudes and as high as 8oC in high latitudes (Higgins and Schrag, 2006). But what was the driver of this ‘fundamental change?’ It has been well understood in marine geology for more than 20 years that the early Eocene was a time of plate rearrangement (Rea et al., 1990). During the Eocene, plate rearrangement accounted for a one to two order of magnitude increase in hydrothermal activity recorded in both north and south Atlantic drilling sites (Rea et al., 1990). In addition, north Atlantic stratigraphy also indicates that the Norwegian-Greenland Sea opened, which coincided with the eruption of the east Greenland volcanoes (Roberts et al, 1984).
Figure 2. Shows a flux at site DSDP 549 in hydrothermal materials on the Y-axis vs. time (in Ma) on the X-axis. This change corresponds to the Paleocene-Eocene boundary (~57Ma). (Rea, et al., 1990)
Site DSDP 549 (figure 2.), from the Deep Sea Drilling Program, of the north Atlantic recorded and increase in CaCO3, Fe, and Mn at approximately 57 Ma, a time that correlates with the recorded benthic extinctions and an increase in global temperatures (Rea et al., 1990).
Kennett and Stott (1991) go on to explain that the Paleocene-Eocene boundary contains one of the largest negative changes in the Cenozoic of δ13C, as well as δ18O recorded in planktonic foraminifera. The research implies that the rapid warming experienced during end of the Paleocene was due to a dominance of warm saline deep waters formed at mid-latitudes, leading to a reduction of cooler high latitude deep waters which would cause a more isoclinal thermal distribution and slow THOC (Kenette and Stott, 1991). Kennette and Stott (1991) propose that tectonic rearrangement and increased hydrothermal activity could rationalize the changes in δ13C measured in foraminifera, but say the brevity and speed at which the recorded change and extinction occur do little to support this hypothesis. On the contrary Rea et al, (1990) hypothesized that sea-floor spreading paired with increased tectonic activity and CO2 release, induced the global warming experienced at the P/E boundary. Whether or not tectonics had an effect of THOC is debatable, however I maintain that multiple tectonic events leading to a high enough release in CO2 and the associated biotic changes on land and within the sea could have altered THOC within the ~6-30ka period.
Similar to the P/E boundary, the mid-Cretaceous was a period of active tectonics, volcanism, and high seafloor spreading rates (Poulsen et al., 2001). This period experienced major physical, biological and chemical changes within the marine system (Poulsen et al. 2001). Evidence for such alterations in the mid-Cretaceous oceans include: deposition of black carbon-rich pelagic shales, major carbon isotope enrichment, a general decrease in oxygen isotopic values, and the expansion and contraction of Caribbean reef lines (Poulsen et al, 2001). Moreover, this era experienced the highest rates of seafloor spreading and off-ridge volcanism than any other time since (Poulsen et al., 2001). It has been estimated that pCO2 during this time ranged from 2 to 6 times higher than modern levels (Cerling, 1991; Freeman and Hayes, 1992; Berner, 1994). Poulsen et al. (2001) interprets these high levels of CO2, a result of increased tectonics, as the driver for the much warmer and less dense ocean waters.
Unlike modern geography, the Cretaceous was characterized by two ‘super continents’, which dramatically influenced the distribution of global circulation pathways. During this time, from ~113-90Ma, a continental arrangement (figure 3.) existed in which North America and Eurasia were still attached, as well as South America and Africa, but the two super continents were not attached to one another leaving two main bodies of water in the Northern and Southern Hemispheres (Poulsen et al., 2001).
Figure 3. A) indicates the Albian arrangement (113-100 Ma) in which Africa and South America were still attached. B) Indicates the Turonian arrangement (93.9-89.8 Ma) where most of the major continents recognizable today had separated creating new oceans basins. Different shades of Grey represent drainage divides within each continent. (Poulsen et al., 2001)
Using the Parallel Ocean Climate Model, this experiment sought to discover how global ocean circulation was driven during a time of such drastic changes in paleogeography and high CO2 concentrations (Poulsen et al., 2001). Although the modeling isn’t perfect, it does give an approximate look into how paleo-ocean currents could have moved based on the constraints of known paleogeography and atmospheric conditions. Modeled results indicated that when increasing CO2 levels (up to 4 times modern levels), thermohaline circulation persisted; however, high latitude deep-water formation diminished (Poulsen et al., 2001). Poulsen et al. (2001) also suggest that the period of plate transition from Albian to Turonian (in simulation) led to significant changes in surface and deep ocean circulation within the north and south Atlantic basins. While not perfect, and still considered a work in progress by the author, this research does show that tectonic adjustments and CO2 composition play an important role in the forcing of global ocean circulation.
In summary, we have explored the basic principles of thermohaline overturning circulation, its defining characteristics, and illustrated how seawater is transported globally based on modern atmospheric and geographic conditions. Research has demonstrated that THOC can change in a variety of ways, including:
- A closing (or opening) of oceanic flow pathways, which disturb internal mixing, density driven circulation, and sea surface temperature.
- Increases in hydrothermal venting and volcanic eruptions which increase atmospheric CO2 levels and tend to drive a positive feedback on global climate, and changing seawater characteristics by modifying acidity (Ph), temperature, and density.
- Sea floor rifting which affects ocean depth and subsequent shoaling along continental margins contributing to salinity and temperature increases in these regions.
We have recorded changes of THOC within the P/E boundary, mid-Cretaceous, and late Neogene. Some of the recorded processes have been thought to have occurred within as little as 6,000 to 30,000 years, while overwhelming evidence shows that usually these changes happen on a timescale well into the millions of years. Records from these studies have come from a variety of locations including the Caribbean, Iceland, north and south Atlantic oceans, Antarctica, and the Pacific ocean, indicating these changes are global.
As previously stated, the current configuration of continents is what helps determine modern ocean circulation. It has been shown that paleo-oceanography has been different in the past, and time will make certain that it will change again in the future. It seems clear from current research that tectonic activity has the potential to greatly manipulate ocean circulation. However, from this discussion we reach a more important question to be addressed in the future: does an alteration in THOC precede global climate change, or is global warming the change needed to drive a shift in THOC?
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