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Australia: The Land Where Time Began |
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Antarctic Oligocene Glaciation – Early
Oligocene Glaciation Preceded by Export of Nutrient-Rich Northern
Component Water The onset of the
formation of North Atlantic Deep Water is believed to have coincided
with the growth of the Antarctic ice sheet about 34 Ma. This timing is
debated, however, partly due to questions about the geochemical
signature of the ancient Northern Component Water (NCW) that formed in
the deep North Atlantic. In this paper Coxall et
al.
present detailed geochemical records from sediment cores obtained from
the North Atlantic close to sites where deep water is being formed. It
was found that the northwestern
Atlantic was
stratified prior to 36 Ma, with bottom waters that were rich in
nutrients and of low salinity. This restricted the transition of the
basin into a conduit for Northern Component Water that began flowing to
the south about 1 Myr prior to the initial glaciation of
Antarctica. It was
suggested by Coxall et
al. that
the probable trigger was tectonic adjustments in subarctic seas that
allowed an increase in the exchange across the Greenland-Scotland Ridge.
The Northern Component Water was strengthened by the increasing salinity
and density of the surface water. The carbon isotope signatures of the
Late Eocene deep
water mass differed from modern values as a result of the leakage of
fossil carbon from the Arctic Ocean. A transient pulse of CO2
to the Earth system was provided by the export of nutrient-rich water,
which possibly caused short-term warming, though the long-term effect of
enhanced formation of Northern Component Water was an increased
transport of heat to the north that cooled Antarctica. In the North Atlantic Ocean the production of
deep water plays a vital role in maintaining the global meridional
overturning circulation (MOC). The North Atlantic Deep Water (NADW),
which is the lower branch of the Atlantic part of the MOC (AMOC),
forms as surface waters cool and densifies in the Labrador and Nordic
Seas. Interplay of:
1)
the stratification at convection sites, which is determined by the
balance of warm salty water from low latitudes, cold fresh water from
the Arctic Ocean and local heat and fluxes of freshwater, and
2)
Upwelling in the Southern Ocean that is driven by wind, by which deep
water is returned to the surface (de Boer, Toggweiler & Sigman, 2008;
Toggweiler & Samuels, 1995). It is probable that both factors impacted
the MOC state in the Early Cainozoic, when bathymetry and ocean gateways
of the Atlantic were different and global temperatures were warmer than
at the present (Abelson & Erez, 2017; Cramer et al., 2009). It is, however, challenging to resolve the
interplay at the onset of the production of North Atlantic Deep Water,
which is referred to in this paper as its palaeo-precursor Northern
Component Water (NCW), as the early history of the AMOC has remained
poorly constrained. The development of
global deep water circulation is constrained by the benthic formation of
δ18O
and δ13C
records by allowing insights into surface temperatures, salinity and
nutrients (Cramer et al., 2009). It is widely believed that the Northern
Component Water began filling the Atlantic Ocean close to the
Eocene-Oligocene greenhouse-to-icehouse climate transition (EOT), at
about 34 Ma (Abelson & Erez, 2017; Davies et al., 2001; Egloff &
Johnson, 1975; Miller & Tucholke, 1983; Via & Thomas, 2006) or earlier
(Borelli, Cramer & Katz, 2014; Boyle et al., 2017; Hohbein, Sexton &
Cartwright, 2012; Langton et
al.,
2016). It has been argued by others that the emergence of a significant
Northern Component Water was delayed until the Miocene (Wright & Miller,
1996). According to Coxall et
al.
modelling studies also diverge, with different studies suggesting that
during the greenhouse-to-icehouse there was no Northern Component Water
(Sijp, Miller & Huber, 2011), a strengthening/onset of the formation of
bipolar deep water that was triggered by the deepening of Drake Passage
(Elsworth et al., 2017; Tigchelaar, von der Heydt & Dijkstra, 2010;
Zhang et al. 2011) or an ocean in which there was a Northern Component
Water throughout (Huber & Sloan, 2001). It was assumed by previous data
studies that argued for an onset of the production of Northern Component
Water in the Late Eocene, that the Early
Cainozoic Northern
Component Water was nutrient poor with a high (‘young’) benthic δ13C
signature that was similar to that of a modern North Atlantic Deep Water
that was well ventilated (Abelson & Erez, 2017; Borelli, Cramer & Katz,
2014; Langton et al., 2016; Pusz, Thunell & Miller, 2011). Yet there is
a lack of palaeodata that is required to characterise the Northern
Component Water close to source regions. Coxall et
al.
produced EOT benthic δ13C
and δ18O
records from deep sea sites at more than 1,000 m palaeodepths in the
high latitudes of the North Atlantic in order to fill this gap. The most
northerly EOT sequence that contained the calcareous microfossils that
were necessary for δ18O
and δ13C
analysis of these is Site 647 in the southern Labrador Sea (SLS) (47oN,
palaeolatitude from 34 Ma, about 2,000-3,000 palaeodepth). For portions
of the Late Eocene additional data were generated from the Deep Sea
Drilling Project (DSDP) Sites 112 and 612, and integrated Ocean Drilling
Project (IODP) Site U14112. According to Coxall et
al.
Northern Component Water export should be recorded by the latter 2 sites
in the deep western boundary current (DWBC). The data were compared with
an isotope compilation that incorporated 14 sites from the Atlantic that
were recorded previously. Records of benthic foraminifera Mg/Ca,
fish-tooth neodymium (Nd) that were reported in epsilon notation (εNd)
and planktonic foraminifera δ18O
and δ13C
that had been obtained from Sites 647 and U1411 were also generated in
order to provide constraints on the bottom-water temperature (BWT) and
provenance, and the structure of the water column. Unique perspectives
are added by these results on the Atlantic end-member deep-water
properties and circulation changes in the circulation during EOT.
Late Eocene Southern Labrador Seas, high nutrient content of its deep
waters Coxall et
al.
recognised at site 647 the typical pattern of increase of δ18O
and δ13C
(>1.0‰ Vienna PeeDee Belemnite (VPDB) and ⁓0.5-1.0‰ VPDB, respectively)
between 34 and 33.5 Ma, which is diagnostic for Antarctic glaciation in
the Early Oligocene, and included in it the peak in δ18O
that is seen at other sites (Coxall & Wilson, 2011), referred to in this
paper as the Early Oligocene Glacial Maximum (EOGM). At site 647 the
first novel observation is that prior to about 35.8 Ma δ13C
of the SLS bottom water was, on average, 0.2-1‰ VPDB lower than at all
the southerly sites. This is the opposite to the modern AMOC state, in
which the northern deep waters have the highest δ13C
as the result of the sinking of surface waters that are well-ventilated
and nutrient poor (Kroopnick, 1985). Coxall et
al
suggest the low value of δ13C
may be a reflection of the nutrient accumulation under stratified
conditions, which is analogous to those of the modern Northern
Pacific, i.e., the
end of the circulation path. It could be implied by this that deep water
that was southern sourced filled the southern Labrador Sea during the
Late Eocene. At site 647, however, fish debris εNd, which is an isotopic
tracer for the origin of deep water masses, bear the fingerprint of a
source in the Northern Hemisphere (εNd = -11.4 to -9.4) throughout the
stratified interval. Coxall et
al.
argued, consequently, that the low benthic δ13C
reflects local bottom water that was sourced from surface waters that
had a high concentration of nutrients within a narrow, restricted
basinal deep water circulation in the North Atlantic. ‘Fossil’ carbon
that leaks from the
Arctic Ocean and subarctic Sea (Greenland Sea and Norwegian Sea) is
a probable source of nutrients, which had high stocks of nutrients
during the Eocene as a result of their semi-isolation, heavily vegetated
margins (Golovneva, 2000) and high riverine inflow (Akhmetiev &
Beniamovski, 2009; Gleason et al., 2009; O’Regan et al., 2011).
1)
Regime 1. Based on the new Eocene-Oligocene
proxy records of Coxall et
al.,
and comparisons with published data, they identify 3 circulation
regimes. Focusing first on δ13C,
under Regime 1(>35.9 Ma), southern Labrador Sea bottom waters were
isolated from the remainder of the Atlantic and a distinct Northern
Component Water that had a low δ13C,
bathed Site 647. After about 35.8 Ma there was a negative δ13C
excursion (0.5-1‰ VPDB δ13C
that continued for about 1.5-2 Ma, from about 35.8 to about 33.8 Ma,
that was seen to varying degrees at sites 612, U1411 and 647, as well as
some other sites in the Atlantic (1053, 1090 and 366).
2)
Regime 2. The timing of offsets between sites
is probably caused by age-model differences. The onset and peak of the δ13C
excursion are encompassed in Regime 2. It is important that the δ13C
excursions are largest, with a maximum of 1‰ VPDB, at sites 612 and
U1411 which is in the Deep Western Boundary Current. The wider
significance of this δ13C
excursion has not been explored fully, though it has been noted
previously (Pusz et al., 2009). The observation that Sites 612 and
U1411, which are directly downstream of Site 647, gain benthic δ13C
signals close to the Sothern Labrador Sea end member suggests the signal
was propagated from the north. Therefore, it records the export to the
south of Northern Component Water from the Arctic imprinted water, which
was nutrient-rich. It is indicated by the increase towards the end of
Regime 2 in Atlantic benthic δ13C
that either the pulse of Northern Component Water ended, or surface
water that was ventilated sufficiently with a higher δ13C
was important to the convection sites.
3)
Regime 3 Regime 3, which is described below, represents the phase during
which a form of Northern Component Water that is more mature existed.
Deep Water cooling, salinification and stratification The pattern of the
Southern Labrador Sea benthic δ18O
is a second prominent feature. During Regime 1in the Southern Labrador
Sea benthic δ18O
is 1-3‰ VPDB lower than that of the ensemble. The primary controls on
the benthic δ18O
are Bottom Water Temperature and the δ18O
composition of seawater – the latter is a reflection of the global
glacial ice volume and local salinity. It is indicated by the relatively
low δ18O
in the benthos in the Southern Labrador Sea that, assuming minimal ice
prior to 34 Ma, a considerably warmer or fresher water mass in contact
with the seafloor compared to the southern stations. From Sites 647 and
112 Benthic δ18O
gradually increased from 36.0 to 35.4 Ma, and again from about 34.6 to
34.4 Ma, and had converged close to the dominant Atlantic trend by about
34.3 Ma, i.e., coincident with or just following the δ13C
minimum that was Atlantic-wide. Diagenetic alteration of Site 647
benthic fossils (Arthur et al., 1989) is ruled out because of:
1)
The excellent fossil calcite preservation,
2)
The similarities of the new values of
planktonic δ18O
from Sites 647 and U1411. Also, a similar
pattern of benthic δ18O
that is decreasing is present in the North Sea record (Nielsen et al.,
2009), though at a shallower depth of about 500m. Bottom Water
Temperature Mg/Ca helps to deconvolve the temperature and salinity
influences on δ18O.
Across the Eocene-Oligocene transition from greenhouse-to-icehouse a
Bottom Water Cooling of ⁓1oC
is suggested by Mg/Ca data at Site 647 Mg/Ca data combined with a ⁓0.6oC
VPDB increase in seawater δ18O
(δ18Osw),
which is in agreement with previous studies (Lear et al., 2008). Coxall
et
al.
estimate there is a gradual cooling of bottom water in the Southern
Labrador Sea of ⁓3-4oC
between 37.5 and 35 Ma, which is similar to that of the northern high
latitude cooling of the sea surface (Liu et al., 2009). By substituting
the Mg/Ca Bottom Water Temperatures into a palaeotemperature equation
yields ice-free δ18Osw
estimates of between -3 and -4‰ δ18O
during this interval. It was estimated, based on these δ18Osw
constraints, that the Late Eocene Southern Labrador Sea bottom
salinities were relevant modern δ18Osw-salinity
of the ocean surface salinity (SSS) relationships (Waddell & Moore,
2008). Applying a modern sea
surface temperature-δ18Osw
relationship from eastern Greenland, which is at the present a conduit
for low salinity water (32 practical salinity units (PSU)) Arctic flow
implies the Bottom water salinity at Site 647 of 30-32 PSU prior to 36
Ma, which increase by 2-3 PSU between 36-34 Ma. The change in salinity
is similar to what occurs when a Laptev Sea, which is at present fed by
large Siberian Rivers, δ18Osw-salinity
regression is applied. The values inferred by
Coxall et
al. are
compatible with modern temperature-salinity fields, though Sea Surface
Salinity-δ18Osw
relationships are widely variable spatially, and relationships of the
present are only loose analogues for the Eocene. Coxall et
al.
therefore suggest that:
1)
The Southern Labrador Sea bottom waters were relatively fresh, and
2)
The Southern Labrador Sea salinity of the bottom waters increased from
Regime 1 to regime 2. Even if samples that
are older than 35 Ma are biased to higher Mg this conclusion does not
change as the salinity signal is buried in the benthic δ18O,
which is independent of Mg/Ca. This interpretation is consistent with
that for nutrients that were pre-formed as they both likely derived from
the Arctic Ocean. It is agreed by proxies and models that during the
Palaeogene there was a thick freshwater cap on the Arctic Ocean as a
result of a strong hydrological system and inputs of fluvial water under
greenhouse forcing which was combined with a salt input that was
restricted (Gleason et al., 2009; Waddell & Moore, 2008; Stärz et al.,
2017; Roberts, LeGrande & Tripati, 2009). As there was no opening to the
Pacific Ocean at this time, major surface discharge occurred through the
Nordic Seas (Stärz et al., 2017; Brinkhuis et al., 2006). Planktonic
foraminifera δ18O
and δ13C
at Site 647 add information on the upper part of the water column. The δ18O
of species from the upper layer (surface) is 1-2‰ VPDB lower than
species from deep water (subthermocline), which is consistent with an
upper ocean that is stratified and calcification of species from the
mixed layer high in the water column or during the warmest season. The δ18O
of the planktonic species from deep water is indistinguishable from the
benthic foraminifera, which is a reflection of deep water that is
relatively fresh at subthermocline levels of the Southern Labrador Sea.
Planktonic data from Site 647 are sparse prior to 34 Ma due to the low
abundance of foraminifera as well as coring gaps. After 34.5 Ma,
however, deep-dwelling planktonic and benthic δ18O
the records separate coincident with the appearance of deep water that
had temperature and salinity properties that were similar to values from
the Atlantic Ocean. Also, it is documented that there was a progressive
collapse of the planktonic-benthic δ13C
gradient that captures the water column transition from being
well-stratified with large vertical δ13C
differences (1-1.5VPDB) during Regime 1, to a state with a smaller δ13C
gradient (0.5‰ VPDB) comparable to the modern North Atlantic convection
sites (Kroopnick, 1985) that were better-mixed by about 34.3 Ma. Both
observations are consistent with increasing volume of the Northern
Component Water. An abrupt shift in the benthic assemblages from Site
647 at 34.3 Ma from agglutinated species tolerant of carbonate-poor,
nutrient-rich environments, to calcareous species that were suited to a
stronger flow of the current (Kaminski & Ortiz, 2014), which was
coincident with other changes, provides further evidence that convection
increased. Circulation Regime 3 began at 34.3 Ma, when a form of
Northern Component Water that was saltier and denser, with a higher δ13C
is exported through the Southern Labrador Sea.
Deep-water sources and sinks The fish debris εNd
data from Site 647 that was obtained by Coxall et
al.
behave as a conservative tracer for northern sourced deep water and it
is possible to compare it with ocean references that have been published
(Burton et al., 1997; O’Nions et al., 1998) in order to identify the
probable source of Northern Component Water. The sample obtained that
dated to 39 Ma was similar to the remainder of the record, which implies
there was no systematic change in εNd and therefore the bottom water
provenance Northern Component Water evolved, though they did not
reconstruct Nd directly from Regime 1. It is suggested by the comparison
that the Southern Ocean,
which has the highest end-member εNd signature in the compilation of
Coxall et
al. Also,
they did not find evidence for the prominent shift to higher εNd values
that were found in the records of the Southern Ocean (Scher & Martin,
2006). It has been suggested by previous studies that, prior to the
deepening of the Greenland-Scotland Ridge, Northern Component Water had
its source in the Labrador Sea (Borrelli, Cramer & Katz, 2014; Langton
et al., 2016). Deep water in the Labrador Sea of the present, however,
has εNd that is characteristically low, about -14), which reflects the
erosional inputs from the cratonic hinterland (Lambelet et al., 2016).
Site 647 Nd, in contrast, is significantly more radiogenic (εNd = -11.4
to -9.4), and is closer to the range of values that have been measured
in the overflows of the Nordic Sea (εNd of about -12.0 to -8.4)
(Lambelet et al., 2016; Lacan & Jeandel, 2005) and above 500 m in the
proximal Arctic Basins (εNd = -11.7 to -8.8) (Porcelli et al., 2009). In
the Palaeogene a
presence of deep water at Site 647 that was sourced to the
Tethys Ocean
is another possibility, as the Tethyian
εNd signature (εNd = -10.0 to -0.3) (Grandjean et al., 1987; Stille &
Fischer, 1990) is indistinguishable from that of the North Atlantic
water masses. Palaeogeographic reconstructions, however, suggest that
the exchange of water mass between the European Tethys and the Nordic
Seas was during in the Middle to Late Eocene (Akhmetiev & Beniamovski,
2009; Kharin & Lukashina, 2010), which makes this unlikely. During the Palaeogene
there were only shallow connections between the Arctic Ocean and the
Nordic Seas (O’Regan et al., 2011; Kharin & Lukashina, 2010; Musatov &
Pogrebitski, 20007). The transfer from the Arctic of freshened,
nutrient-rich waters would have occurred by a proto Greenland Current.
The similarity between the North Sea (Kysing-4 borehole) and Site 647
benthic δ18O,
as well as independent evidence of low salinities in the Nordic Seas
(Stärz et al., 2017; Porcelli et al., 2009), is, according to Coxall et
al., consistent with this picture.
Transport was also shallow from the subarctic seas to the Atlantic
Ocean, and the sinking of Northern Component Water that was
Arctic-imprinted must have occurred south of the Greenland-Scotland
Ridge until it subsided. When there is sufficient cooling in the
subarctic seas. The contrast in density between brackish Arctic waters
with warmer, saltier surface waters from the North Atlantic permitted
sinking, with the result that at Site 647 distinct bottom water is
recorded. It is important that
Arctic-imprinted Northern Component Water formation was minimal prior to
about 36 Ma, which implies there was a regular stratification and
stagnation in the Southern Labrador Sea. According to Coxall et
al.
this is consistent with the considerable noise in δ13C
and δ18O
during Regime 1. It has remained uncertain how this deep water has
remained isolated in the Southern Labrador Sea. According to Coxall et
al. it is possible that in the
North Atlantic the production and export of local deep waters were high
compared with the influx of deep waters that were sourced from the
south, subsequently increasing further as the cooler, saltier Northern
Component Water began being produced. There is an alternative
possibility that bathymetric highs that were associated with the now
extinct Labrador Sea spreading ridges and the West Thulean Igneous
Province to the south may have been the cause of the subsurface waters
of the Southern Labrador Sea from the overall Atlantic during the early
Palaeogene (Egloff & Johnson, 1975). In this case, it was probably
important that the cessation of the Labrador Sea spreading close to the
Eocene-Oligocene greenhouse-to-icehouse climate transition, as it
allowed ridges to subside thereby enabling enhanced export of deep
water. In this paper Coxall
et
al.
illustrate the isotopic evidence and sequence of EOT oceanic changes by
use of a natural neighbour regridding of the isotopic data that has been
compiled to produce the south-north depth transects in the Atlantic
Ocean during time windows that were centred on circulation regimes 1-3.
Prior to about 36 Ma, Regime 1, there was a strong isotopic δ18O
and δ13C
depletion that affected water masses down to about 2,000 m above 40oN,
which corresponded to small amounts of Arctic-imprinted Northern
Component Water of low salinity, high-nutrient in the Southern Labrador
Sea. Deep waters that have a more homogeneous δ18O
that was sourced from southerly and possibly regions of low latitude
(Borrelli, Cramer & Katz, 2014; Langton et al., 2016) filled the
remainder of the Atlantic Ocean. A pulse over a period of 0.5-1.0 of
Northern Component Water export during Regime 2 accompanied increasing
subarctic δ18O,
which reflects progressive salinification and densification of Nordic
surface waters. The ‘fresh’ Southern Labrador Sea deep water did not
exist by 33.3-34.3Ma, there was an increase in bottom water δ13C,
and the acute phase of δ13C
was over and a Northern Component Water was exported that was better
ventilated (Regime 3). It is important that the initial pulse of export
of Northern Component Water under Regime 2 is recorded by δ13C
signals downstream of the Southern Labrador Sea that are decreasing in
deep waters. In previous δ13C
records (Borrelli, Cramer & Katz, 2014; Langton et al., 2016; Pusz,
Thunell & Miller, 2011) the presence of Late Eocene Northern Component
Water in the Atlantic Ocean is not identified as it was assumed that
Northern Component Water had a high δ13C
signature that was similar to that of the modern Atlantic Deep Water.
Causes and consequences of Northern Component Water In the Late Eocene
deepening of Greenland-Scotland Ridge (GSR) for which there is
geological evidence (Abelson & Erez, 2017; Stärz et al., 2017), would
have increased the Nordic overflows, and thereby strengthened the
production of Northern Component Water. It is suggested by modelling
that deepening of the Greenland-Scotland Ridge below a depth of 50 m
initiates a threshold switch from lagoonal to estuarine circulation that
salinifies the Nordic Seas enough to cause intensification of the
production of northern deep water (Stärz et al., 2017). The bathymetric
history of the GSR is currently too crude to accurately date such a
change, though this idea is consistent with the findings of Coxall et
al. Also, it is proposed by Coxall
et
al. that
contemporary restrictions to the Arctic-Nordic Sea exchange also played
a role. It is suggested by geological evidence that the Barents
Sea-Arctic passageway shoaled in the latest Eocene (Kharin & Lukashina,
2010; Musatov & Pogrebitskij, 2000) and that variations in relative
sea-level in the Arctic were decoupled from global trends from the Late
Eocene to the Early
Miocene (Hegewald & Jokat, 2013). The salinification of the Nordic
Seas as brackish overflows from the Arctic were gradually cut off
enhanced the palaeogeographic isolation of the Arctic. Changes in the
production of NCW had effects that were varied and competing. It is
assumed that its onset affected polewards transport of heat in both
hemispheres (Tigchelaar, von der Heydt & Dijkstra, 2010; Zhang et al.,
2011). Coxall et
al.
suggest that the initial export of nutrient-rich, Arctic-imprinted NCW
may have generated a pulse of CO2
that was short-lived on the order of 100-200 ppm, which is consistent
with the proxy compilations that display a temporary reversal in the
dropping CO2
trend between about 34 Ma and about 35 Ma (Anagnostou et al., 2016). A
strengthening of the production of NCW and enhanced ocean heat transport
to the north, on the other hand, could have played a role in the
drawdown of CO2
in the long term as a result of an accompanying rainfall increase over
land and associated feedbacks with CO2-weathering
(Elsworth et al., 2017). The change in timing of the circulation change,
1-2 Myr before the glaciation of Antarctica, reinforces the idea that a
role was played by the onset of NCW in preconditioning the Earth system
during the Late Eocene for the transition from greenhouse to icehouse.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |