2.3 MÉTODOS DE APRENDIZAJE
2.3.2 La llegada del BIM a la formación universitaria
Present day contourites w ithin the Faeroe Shetland Basin are deposited by alongslope currents w hich flow through the basin as part o f the N orth A tlantic Conveyor Belt
circulation defined by B roecker (1991). The present day flow o f these currents through the basin is driven by four m ain elements: 1) inflow o f w arm saline surface w aters into the N orw egian G reenland Sea; 2) therm al densification and sinking o f the surface waters; 3) outflow o f cool deep w aters into the N orth Atlantic; and 4) the presence o f a deep w ater gatew ay betw een the N orw egian G reenland Sea and the N orth A tlantic (Nilsen, 1983;
B roecker and Denton, 1990; Broecker, 1991;Schmitz, 1995; W right and M iller, 1996;
Hansen and 0sterh u s, 2000). The evidence for the existence o f each o f the com ponents during the Eocene is considered in turn below.
2.8.1.1. D epositional current
The identification o f the middle Eocene drift necessarily im plies the action o f bottom currents at this time that w ere part o f the therm ohaline or m ajor w ind driven circulation pattern o f the oceans (Stow et al., 2002). The flow direction o f the depositional current can be constrained from the tendency for contourite drifts to m igrate dow nstream and alongslope (Faugeres et al., 1999), w hich is represented seism ically as a progressive axially directed dow nlap/onlap (depending on the angle o f the surface onto w hich the reflections lap out). In
2-39
Chapter Two Eocene Contourites
a younger drift body (the SE Faeroes Drift) located to the northeast along the base o f the Faeroese slope, southerly directed dow nlap o f internal drift reflections has been used to argue for the onset o f deep w ater circulation b y southerly flow ing currents during the Early O ligocene (Davies et al., 2001). The identification o f southerly directed progressive onlap o f internal reflections o f the P30 drift onto the inclined IEU at the southern end o f the Judd Basin is likewise interpreted to have been deposited by a southerly flowing depositional current, but m uch earlier than suggested by D avies et al., (2001).
This southerly flow ing w ater m ass was evidently in place for a long period o f relatively stable flow, judging from the consistency o f the onlap w ithin the c.800m thick P30 drift body. This drift is interpreted to have been deposited in w ater depths o f >500m, im plying that the southw esterly flow ing w ater m ass w as linked to deepw ater circulation rather than w ind-driven surface w aters. D ating o f the P30 drift body shows that deposition initiated during the m id-Eocene (Lutetian - base - 49M a) and continued until at least the Late Eocene as recorded b y the Priabonian taxa A.M ichoudii w ithin well 213/23-1 (nannofossil zone NP18 dated as 37-36M a, Berggren et al., 1995).Therefore, the c.800m o f sedim ent w ere deposited over approxim ately 12-13Ma, resulting in an average sedim entation rate o f 6-7 cm/ka. This value corresponds w ell to average sedim ent accum ulation rates on elongated m ounded drifts o f 2-10 cm /ka (Stow et al., 2002).
H aving established the onset and longevity o f the drift body, the question arises to w hat extent this drift w as the product o f localised circulation w ithin the Faeroe Shetland Basin, or a through-going circulatory regim e, linking the nascent N orw egian G reenland Sea w ith the m ain Atlantic O cean via the gatew ay o f the Faeroe Shetland Basin? Put differently, w as the Faeroe Shetland Basin open as a gateway, and w hat proportion o f deep w ater exchange betw een the N orw egian Sea and the A tlantic w as routed through this gateway?
2.8.1.2. Deep water connection
A physical connection between the N orw egian G reenland Sea and the N orth Atlantic is critical for the exchange o f deep w ater m asses as part o f the N orth Atlantic Conveyor Belt circulation (Broecker, 1991; W right and M iller, 1996). A t present the G reenland Scotland Ridge (GSR) forms a barrier betw een the N orw egian G reenland Sea and the N orth A tlantic,
2-40
Chapter Two Eocene Contourites
w ith the passage o f deep w aters form ed in the N orw egian G reenland Sea perm itted in 3 locations: the D enm ark Strait, the Iceland-Faeroe Ridge and m ost significantly the Faeroe Shetland Gateway, com prising the Faeroe Shetland Basin and the Faeroe Bank Channel (Figs. 2.1 & 2.2, Hansen and 0sterhu s, 2000). Therefore, in order for a m odem style through going current circulation to have been responsible for deposition o f U nit P30 a physical deep w ater connection betw een the basins w ould have had to existed across the G SR from the m iddle Eocene onward. To assess this possibility, it is critical to understand the tectonic developm ent o f the G SR and the Faeroe Shetland Gateway.
Structure and evolution o f the G reenland Scotland Ridge
The G SR extends from the SE continental m argin o f G reenland to the N W continental m argin o f Scotland, form ing a m ajor bathym etric ridge 4000km long and up to 1500km w ide (at the Iceland plateau) w hich separates the N orth A tlantic from the N orw egian G reenland Sea (Fig. 2.1, Bott, 1983; N ielsen, 1983). The ridge com prises 3 m ain sections w hich are geologically distinct: 1) the Icelandic transverse ridge; 2) the Faeroese Block and 3) the W yville Thom pson Ridge (Bott 1983).
The earliest com ponents o f the G SR appear to be the Faeroe Platform and associated m inor platform s (Faeroe Bank, Bill Bailey Bank and the Lousy Bank). The arrival o f the proto- Icelandic plum e at c.62M a resulted in the onset o f extrusive volcanism in the region (W hite and Lovell, 1997; Sorensen, 2003; Lam ers and Carm ichael, 1999; Ritchie and Hitchen,
1996; Pearson, 1996; Jolley and Bell, 2002). Peak rift m argin m agm atism w as reached at c.58M a (W aagstein, 1988; W hite and M cKenzie, 1989; Bell and Jolley, 1997; Jolley, 1997;
Sorensen, 2003) w ith extensive extrusion along the continental m argins o f central w est Greenland, East Greenland, Rockall, Faeroe Platform , V oring Plateau, NW Scotland, N Ireland and w ithin the Faeroe Shetland Basin (N aylor et al., 1999). The Faeroe platform, w hich consists o f 5km o f basalt overlying rifted continental crust, was created between 59- 55.5M a (W aagstein, 1988; Lamers and Carm ichael, 1999; N adin et al., 1999; N aylor et al.,
1999; A ndersen et al., 2002; Sorensen, 2003).
Full oceanic spreading in the northern N orth A tlantic com m enced in the earliest Eocene (Roberts et al., 1999), w ith N-S com pression associated w ith spreading on the A egir ridge
2-41
Chapter Two Eocene Contourites
north o f the Faeroe Shetland Basin believed to be responsible for form ation and early grow th o f the W yville Thom pson Ridge during that tim e (Boldreel and Andersen, 1993; A ndersen and Boldreel, 1995; Dore et al., 1997). This com pression was also responsible for form ation o f the M unkagrunnar Ridge and the Judd A nticline, am ong other associated features
(Boldreel and Andersen, 1993; Sorensen, 2003), w hich led to the definition o f the Faeroe- Bank Channel in its present day form.
Ridge spreading occurred both south (Reykjanes Ridge) and north (Jan M ayen Ridge) o f the G SR from the early Eocene, and form ation o f the Icelandic transverse ridge is thought to have occurred sym m etrically in harm ony w ith the R eykjanes ridge (Bott, 1983). This spreading ridge w hich form ed the Iceland-G reenland and Iceland-Faeroe Ridges probably stood at 1.5-2km above sea level at the tim e o f formation, w ith the oldest parts sinking below sea level c.20M a, based on estim ation o f subsidence rates (Vogt, 1972; Bott, 1983).
The Icelandic transverse ridge is thus the youngest part o f the ridge, and its formation is dated from m agnetic anom aly 24 (55.9-52.36M a) to present (Bott, 1983; Hardenbol et al.,
1998).
The history o f the GSR can be sum m arised as fo llo w s:
1. Palaeocene volcanism associated w ith im pingem ent o f the Proto-Icelandic Plume began at 62M a and intensified at 58M a resulting in extensive basalt extrusion, and form ation o f the Faeroe Platform
2. Initiation o f seafloor spreading north o f the Faeroe Shetland Basin during the early Eocene resulted in N-S com pression and form ation o f the W yville Thom pson Ridge, M unkagrunnar Ridge and Judd anticlines in the southern Faeroe Shetland Basin.
Creation o f the W yville Thom pson R idge and the M unkagrunnar Ridge defined the new ly formed Faeroe-Bank Channel.
3. Spreading o f the N orth A tlantic R idge w hich began in the early Eocene and is ongoing resulted in creation o f the Iceland-Faeroe-Ridge, w hich forms the youngest section o f the GSR.
2-42
Chapter Two Eocene Contourites
Evolution o f Faeroe Shetland Basin
D uring the early to m id Palaeocene the Faeroe Shetland Basin existed as a deep w ater basin at the southern end o f the G reenland Scotland R idge (Lam ers and Carm ichael, 1999;
Sm allw ood and Gill, 2002). During this tim e the basin is thought to have been open to the south into the Rockall Trough because W yville Thom pson Ridge did not exist in its present day form. Shoaling o f the southern Faeroe Shetland Basin as a result o f plum e uplift
eventually resulted in em ergence and form ation o f the base Balder unconform ity during the late Palaeocene/early Eocene (Sm allw ood and Gill, 2002). The southern Faeroe Shetland Basin w as subsequently drow ned during the early Eocene, and deep w ater conditions
(>450m) w ere restored throughout the axis o f the basin by the Lutetian. As discussed above, the Faeroe Bank Channel was also defined in the early Eocene.
The evolution o f the F aeroe Shetland Basin can be sum m arised as follow s:
1. Deep w ater conditions prevailed throughout the basin during the early to m id Palaeocene
2. U plift in association w ith the proto-Icelandic Plum e resulted in relative sea level fall and em ergence and shallow w ater conditions prevailed across the southern Faeroe Shetland B asin during the Late Palaeocene and Early Eocene. M eanwhile, deep w ater conditions (>500m ) w ere m aintained tow ard the north o f the basin.
3. Follow ing the initiation o f sea floor spreading in the N orth Atlantic during the Early Eocene, the southern Faeroe Shetland Basin experienced subsidence and relative sea level rise, resulting in the re-establishm ent o f deep w ater conditions throughout the basin axis by the M iddle Eocene.
Therefore, analysis o f the developm ent o f the G reenland Scotland Ridge and the Faeroe Shetland Basin reveals that by the early Eocene the Faeroe Shetland Basin constituted a deep w ater gatew ay across the new ly form ed G reenland Scotland Ridge. The developm ent o f the W yville Thom pson and M unkagrunnar Ridges during the early Eocene provided the Faeroe Bank Channel as a deep w ater exit for w aters flow ing into the Faeroe Shetland Basin from the N orw egian G reenland Sea. However, only one o f the three phases o f com pression reported to have created the present day W yville Thom pson Ridge had occurred before the
2-43
Chapter Two Eocene Contourites
end o f the Eocene, and so it is likely that the ridge w as o f a low er am plitude than at present, thus potentially allow ing the overflow o f deep w aters across it.
2.8.1.3. D eep water fo rm ation
D eposition o f the thick P30 contourite succession over at least 13Ma requires an established southerly flowing deep w ater current through the Faeroe Shetland Basin during this time.
The proposed existence o f the Faeroe Shetland B asin as a deep w ater basin since the early- m iddle Eocene suggests that from this time onw ards a deep w ater connection betw een the N orw egian G reenland Sea and the N orth A tlantic through the Faeroe Shetland Basin w as possible. Therefore, U nit P30 is proposed to have been deposited by southerly flowing overflow w aters likely exiting the Faeroe Shetland Basin into the N orth Atlantic through the Faeroe Bank Channel, w ith the possibility o f som e breaching o f the developing W yville Thom pson Ridge. This w ater m ass is referred to as N orthern Com ponent W ater (sensu Broecker and Peng 1982), and is interpreted as the equivalent o f N orwegian Sea O verflow W ater that flows south through the Faeroe Shetland Gatew ay at present.
A prerequisite for the functioning o f the N orth Atlantic Conveyor B elt circulation is a latitudinal tem perature gradient so as to perm it generation o f cool deep waters at high latitudes (Broecker and Denton, 1990; Rahm storf, 2002). Evidence presented here for a sim ilar circulatory regim e in the m iddle Eocene w ould thus require a significant latitudinal tem perature gradient. The question o f deep w ater production is central to reconstruction o f the climatic regim e driving the therm ohaline circulation in the m iddle Eocene. Deep w ater can be generated via halo-therm al processes, w here the density o f surface waters is increased by evaporation to a point w here it sinks into the deep ocean (Brass et al., 1982; Pak and M iller, 1992; Sloan et al., 1995). This m ethod o f deep w ater form ation requires restricted m arine conditions, low fresh w ater input and high solar insolation (Kennett and Stott, 1991;
Pak and M iller, 1992). However, by the early Eocene the N orwegian G reenland Sea w as a significant m arine basin that straddled 60-70°N (Dore et al., 1999; Lundin and Dore, 2002), having m igrated northw ard throughout the M esozoic and early Cenozoic (Knott et al., 1993).
Eocene deep w ater tem peratures, and thus polar surface temperatures, are estim ated to have been c. 6 -8°C during the early to m iddle Eocene, w ith near m odem tropical sea surface tem peratures (Shackelton and Boersma, 1981; G reenw ood and Wing, 1995; H uber and
2-44
Chapter Two Eocene Contourites
Sloan, 2001; Zachos et al., 2001). The com bination o f a high northern latitude, open m arine basin and a latitudinal tem perature gradient suggests that deep w ater form ation north o f the G reenland Scotland Ridge during the early to m iddle Eocene was likely the result o f therm al densification as a result o f loss o f heat to the atmosphere, and not halo-therm al processes.
Sinking o f surface waters w ould be m ost likely to have sustained the southerly deep w ater flow and contourite deposition in the Faeroe Shetland Basin during the m iddle Eocene.
In order to sustain the sinking and export o f deep waters through the Faeroe Shetland Basin, a northw ard inflow o f surface w ater into the N orw egian G reenland Sea w ould be required in order to replenish w aters rem oved from the surface during deep w ater production, as occurs at present (Hansen and 0 sterh u s, 2000). Eocene contourite deposits identified by seismic stratigraphic analysis o f the outer Voring M arginal High w est o f N orw ay are interpreted to have been deposited b y northw ard flow ing alongslope currents (Laberg et al., 2005). This is used as evidence to support a northw ard surface inflow into the N orw egian G reenland Sea during the Eocene, and suggests that all o f the criteria, w hich w ould have been necessary for a m odem style N orth A tlantic Conveyor Belt circulation to have been operational in the m iddle Eocene, w ere present.
Evidence for dram atically shifting climatic and oceanographic regim es during the Eocene has m ounted in recent years, and provides support for the notion that the m odem -style o f therm ohaline circulation m ay have initiated in the M iddle Eocene. M ajor climatic cooling betw een the m iddle Eocene and the O ligocene resulting in an increased latitudinal
tem perature gradient w as invoked by M iller and Tucholke (1983) as a potential causal factor in the onset o f deep w ater circulation betw een the N orw egian G reenland Sea and the N orth A tlantic during the late Eocene-early Oligocene. Large erratic fluctuations in atmospheric CO2 levels throughout the Eocene cooling are interpreted as evidence o f a less stable climatic regim e during the late Palaeogene com pared to the N eogene (Pearson and Palmer, 2000), and changes in the palaeoceanographic circulation o f the W estern N orth Atlantic during the middle Eocene also point tow ard fluctuation in oceanic circulation stability (W ade and Kroon, 2002). Furtherm ore, recent sedim entological and geochem ical analysis o f Pacific and Southern A tlantic sedim ent cores has been used to propose the existence o f large northern hem isphere ice sheets during the M iddle Eocene (Tripati et al., 2005). This claim is
2-45
Chapter Two Eocene Contourites
supported by the discovery o f ice rafted debris in m id Eocene cores from the Lom onosov Ridge in the A rctic O cean (ACEX, 2005). The proposal o f northern hem isphere ice sheets during the M iddle Eocene is highly controversial (Kump, 2005; C. Lear, pers comm.), but provide potential evidence o f polar cooling w hich can be used to support our inference o f cool atm ospheric conditions in the region during the m iddle Eocene.