Upload
lewis-hughes
View
242
Download
6
Embed Size (px)
Citation preview
Lewis Hughes - B8470472 – SXG390 – EMA
Abrupt climatic reversal evidenced in Cariaco Basin sediments;
multiproxy analyses to determine processes forcing environmental
change during the Younger Dryas, and implications for
understanding today’s climate.
A report submitted as the examined component of the Project Module SXG390.
Lewis Hughes
B8470472
22nd September, 2015.
Word Count: 4,999
Page 1 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Abstract
Literature surrounding the Younger Dryas period within the Cariaco Basin, Venezuela has
been examined, critically evaluated and synthesized into a literature review. Evidence for
abrupt climate change within the Cariaco Basin at the Younger Dryas onset is found in
lighter colouration and increasing thicknesses of sedimentary laminations measured at 13
ka, signifying higher bulk sedimentation rates. Sharply increased sedimentary radiocarbon
content of up to 35 ppm attests to simultaneous large-scale perturbations in thermohaline
circulation. Increasingly positive oxygen isotope ratios from -0.5 ‰ to 0.5 ‰ and
increasingly negative Magnesium/Calcium ratios from 4.5 mmol/mol to 3 mmol/mol,
suggest decreased sea surface temperatures of 3-4°C. Large scale change in the dominant
phytoplankton community is also apparent, with preserved communities switching from
flagellate dominated, to diatom dominated, consistent with increased upwelling during the
period. Proxy responses collectively indicate abrupt cooling of the Cariaco Basin during the
Younger Dryas, with changes in primary production and bulk sedimentation rates,
concomitant with changes in the hydrological cycle, oceanic circulation and upwelling
intensity. Feedback processes forcing climatic change are found to be southward migration
of the Inter-Tropical Convergence Zone in response to thermohaline shutdown and renewed
ice sheet growth. Southward migration brings the trade winds directly over the Cariaco
Basin, inducing coastal upwelling, enhancing nutrient supplies and sustaining high levels of
primary production and sedimentation, similar to what is seen in today’s winter period. This
report suggests that the preserved record within the Cariaco Basin can be used to examine
and reconstruct how tropical regions respond to rapid climate shifts in the past, and also
help in our understand of the climate today.
(266 words)
Page 2 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
List of abbreviations
Abbreviation Definition10Be Radioactive isotope of Beryllium14C Radioactive isotope of Carbon
CB Cariaco Basin
δ18O Shorthand expression of ratio of 18O isotopes
to 16O expressed towards the mean sea
water standard.
G.ruber Globigerinoides ruber
G.bulloides Globigerina bulloides
ITCZ Inter-tropical convergence zone
ka Thousands of years
Mg/Ca Magnesium/Calcium
NADW North Atlantic Deep Water
OC Organic carbon
ppm Parts per million
SST Sea surface temperature
Uk37 Alkenone unsaturation index, a ratio of
double carbon bonds to triple carbon bonds
within the tests of foraminifera
YD Younger Dryas
Page 3 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Table of contents
Chapter/Section Page Number
Abstract 2
List of abbreviations 3
Table of contents 4
List of tables 5
List of figures 6
1. Introduction. 7
1.1. Scope of work. 7
1.2. Objectives. 8
1.3. Search Methodology. 8
2. Evidence and timing of environmental change in The Cariaco Basin. 9
2.1. The geological setting of the Cariaco Basin. 9
2.2. The sedimentary record of the Cariaco Basin. 9
2.3. Timing the abrupt change. 14
3. Interpreting the evidence preserved in the sedimentary record. 14
3.1. Reconstructing the Younger Dryas environment of the Cariaco Basin. 14
4. Processes and factors forcing abrupt climate change. 16
4.1. External forcing. 16
4.2. Internal forcing, processes and feedback mechanisms. 16
5. Using the Cariaco Basin paleorecord to understand climate today. 19
5.1. What can we learn? 19
5.2. Implications for today’s climate. 20
6. Discussion. 20
7. Conclusion. 22
References 24
Page 4 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
List of Tables
Table Number Page Number
Table 1 – Tabular summary of the differences in the environment 15
of the Cariaco Basin during the Younger Dryas, compared to
the present day. Compiled by Hughes, 2015.
Page 5 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
List of figures
Figure Number Page Number
Figure 2.1 – Location and bathymetric map (contours in metres 9
below sea level) of the CB, location of ODP Hole 1002 indicated.
Modified from Clayton et al (1999).
Figure 2.2 - Greyscale values from core PL07-56PC of the Cariaco basin (a) 10
with the Younger Dryas period visible as the clear minimum in values.
(b) Radiocarbon values from the same core.
Modified from (Muscheler et al., 2000).
Figure 2.3 - Oxygen isotope data derived from G. ruber, white, within 12
Cariaco Basin sediment core PL07-39PC (a) (Lin et al., 1997). Mg/Ca
ratios also derived from G. ruber, white, but with pink variety analysed
where limited abundance occurred (b) (Lea et al., 2003). Both proxies
show a recognisable fall at 550 cm core depth, equivalent to the
YD period. From Lea at al (2003).
Figure 2.4 - Mg/Ca derived SST of the Cariaco basin (Purple) 13
(Lea et al., 2003) and alkenone derived SST from the Caribbean Sea
(green) (Ruhleman et al., 1999). Grey band indicates timing of the YD
period. Modified from Carlson (2013).
Figure 4.1 - Location of the present day ITCZ during summer periods, 17
showing maximum rainfall (a) and Location of the ITCZ during the YD,
showing southerly displacement (b). From Riboulleau et al (2014).
Figure 4.2 - 10Be flux from the Greenland summit core. – Modified 19
from (Muscheler et al., 2000).
Page 6 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
1. Introduction.
1.1. Scope of work.
Climate change is a complex web of interacting processes acting over many timescales, with
abrupt change carrying the potential for mass extinction. Much research is being
undertaken to advance our understanding of the climate, however abrupt, sub-millennial
climatic change and its effects in low latitudes is less well understood than in high latitudes.
Here, only literature surrounding abrupt climate change in the Cariaco Basin during the
Younger Dryas period is examined. Four themes are focused on; describing evidence, its
interpretation, processes identifiable from evidence, and implications for today’s climate.
The Younger Dryas (YD) was a brief period of abnormally cold conditions across the
Northern Hemisphere at ~13 ka (Carlson, 2013), abruptly reversing rapid warming
associated with the last glacial termination. It is the only known example of climatic reversal
during glacial/interglacial transitions. The Cariaco Basin (CB) is a low latitude anoxic basin off
the coast of Venezuela with high primary production rates (Dahl et al., 2004). This allows
high resolution preservation of sedimentary proxies, in one of only a few areas with
resolution matching Greenland ice cores.
Proxy responses suggest that during the YD the CB underwent an abrupt change to arid,
cooler conditions; against some views of little change in the tropics during glacial periods.
However proxies often record more multiple climatic variables in their responses, and by
utilizing multiple proxies such as fossil assemblages, geochemical ratios and sediment
colouration, confidence in the signals increases.
Linking together the proxies’ responses also helps to reveals the processes and mechanisms
forcing climatic change in the past, allowing us to understand how the environment of the
past functioned, whilst also aiding our understanding of the climate today.
Page 7 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
1.2. Objectives.
Objectives of this report are:
1. Describe the evidence for the YD recorded in oceanic sediments in the CB,
Venezuela, through proxies such as varves, 14C content, Mg/Ca and 18O ratios, and
phytoplankton communities.
2. Define the onset timing of the YD in the CB, by dating the proxy responses to past
climatic changes.
3. Interpret the changing proxy responses to infer the past environment of the CB
during the YD, which suggest reduced sea surface temperatures, increased aridity
and changes in dominant primary producers.
4. Account for the processes responsible for forcing climatic change identifiable from
the responses, such as changes in thermohaline-circulation and upwelling from 14C
contents, and increased polar ice sheet cover from 18O values, which reveal how the
environment functioned during the period.
5. Discuss how the processes forcing past climatic change can then help inform our
understanding of the planet and its climate today.
1.3. Methodology.
Literature was found through searching of online databases/journals, via keywords within
the Open University’s online library. Searches began wide ranging i.e. “Younger Dryas AND
North Atlantic” becoming more focused, incorporating keywords found in published
research i.e. “Younger Dryas AND Cariaco Basin NOT Holocene” and “Cariaco* AND (Mg/Ca
OR “Oxygen Isotope)”.
Keywords were then transferred to other online databases such as Science Direct, Web of
Science, Springer Link and Wiley, and E-journals such as Quaternary Science Reviews, and
Journal of Quaternary Studies. Credibility was assessed via the PROMPT method.
Page 8 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
2. Evidence and timing of environmental change in the Cariaco Basin.
2.1. The geological setting of the Cariaco Basin.
The CB lies on the continental shelf of northern Venezuela (Figure 2.1), consisting of two,
deeper sub-basins separated by a shallow saddle. Shallow sills isolate the basin from the
open waters of the Caribbean Sea, restricting deep water exchange (Clayton et al., 1999).
Deep waters frequently become anoxic, promoting excellent preservation of sedimentary
proxies over time (Lin et al., 1997).
Figure 2.1 – Location and bathymetric map (contours in metres below sea level) of the CB, location
of ODP Hole 1002 indicated. Modified from Clayton et al (1999).
2.2. The sedimentary record of the Cariaco Basin.
Various proxies within CB sediments record evidence of abrupt climatic change during the
YD. Excellent preservation is afforded by the higher carbonate ion content of the CB waters
during the YD (Lea et al., 2003), a claim reinforced by Riboulleau et al (2011) who show a
10% rise in carbonate concentrations. Werne et al (2000) add that anoxic conditions aid
preservation further by limiting the process of bioturbation.
There is agreement that during the YD varves show increased thicknesses, from 1 mm to 3
mm (Hughen et al., 1996), with falls in organic carbon (OC) content (Riboulleau et al., 2011)
and reductions in greyscale value from ~200 to ~170 (Dahl et al., 2004; Lea et al., 2003)
Page 9 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
(Figure 2.2a). Varves are annual sedimentary accumulations deposited in marine and
lacustrine environments, forming pairs of light and dark layers that are influenced by
seasonal climate differences (Wilson et al., 2007). Dahl et al (2004) claim the dark/light
banding is influenced by the dominance of mineral rich layers in warm, wet periods, versus
plankton rich layers in cool, dry periods, respectively. Other studies concur (Hughen et al.,
1996); however conflicting views from separate researchers regarding the reason for low OC
exist. Werne et al (2000) cite a consequence of increased plankton production diluting
sediments; whereas Riboulleau et al (2011) contest that a reduced flux of OC is responsible.
Both claims carry merit, and it is my view that further research to elucidate the underlying
process would be beneficial, as fluctuating organic carbon could be used to interpret
changes, if any, in primary production at the surface, and changes in water column
oxygenation.
Figure 2.2 – Greyscale values from core PL07-56PC of the Cariaco basin (a) with the Younger Dryas
period visible as the clear minimum in values. (b) Radiocarbon values from the same core.
Modified from (Muscheler et al., 2000).
Figure 2.2b shows atmospheric radiocarbon (14C) exhibiting a pronounced rise in
concentration within CB sediments during the YD; amounting to a 35 ppm (70%) increase
(Hughen et al., 2000; Muscheler et al., 2000). 14C is a radioactive isotope formed by cosmic
ray bombardment of atmospheric Nitrogen atoms (Wilson et al., 2007). Broecker (2003)
sees elevated 14C concentrations in YD sediments as a consequence of and evidence for
thermohaline circulation reduction, which acts as a 14C sink under present conditions,
removing it to the deep ocean. Goslar and Arnold (2000) however argue in favour of
increased production via enhanced solar activity, evidenced through elevated
concentrations of 10Be, a further radioactive isotope formed identically to 14C, in lacustrine
Page 10 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
sediments. In my opinion, claims made by Goslar and Arnold appear speculative, as a single
locality is being used to argue globally elevated levels of 10Be, and surely must be treated
with caution and backed up with further studies from other localities to obtain a full picture.
This leaves the claims of Broecker of thermohaline shutdown as the most plausible reason
for elevated 14C during the period.
Oxygen isotope ratios (δ18O) of the planktonic foraminifera Globigerinoides ruber, White,
have been analysed by Lin et al (1997). They report that δ18O records a noticeable reversal
during the YD, with increasingly positive values from -0.5 ‰ to 0.5 ‰ (Figure 2.3a). Oxygen
isotope ratios are affected by a multitude of climate variables such as salinity, ice sheet
volume, and temperature of prevailing water (Coe et al., 2005); separating out the various
signals is challenging, but not impossible. Lin et al (1997) argue that their data shows a
reduction in sea surface temperature (SST), but Herbert and Schuffert (2000) contest that
the δ18O variations are satisfactorily explainable by salinity changes. Aspects of Lin’s
methodology are questionable, as Peterson et al (1991) report that G.ruber represented
only 10% of the foraminiferan population during the YD, and low sampling abundance may
carry uncertainty. Tedesco et al (2007) have also reported seasonal variations in species
calcification depths, which could influence the signals recorded, calling for caution when
interpreting δ18O values. Lin et al (1997) however argue that G.ruber is a reliable annual
surface temperature proxy, given its non-association to upwelling events, and Lea et al
(2003) agree, stating G.ruber shows a constant annual distribution.
Page 11 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Figure 2.3 – Oxygen isotope data derived from G. ruber, white, within Cariaco Basin sediment core
PL07-39PC (a) (Lin et al., 1997). Mg/Ca ratios also derived from G. ruber, white, but with pink
variety analysed where limited abundance occurred (b) (Lea et al., 2003). Both proxies show a
recognisable fall at 550 cm core depth, equivalent to the YD period. From Lea at al (2003).
Mg/Ca ratios in foraminiferan tests are a further SST proxy, affected only by the prevailing
water temperature (James, 2005). Lea et al (2003) examined Mg/Ca ratios recorded in G.
ruber, white. Mg/Ca ratios were found to fall abruptly from 4.5 mmol/mol to 3 mmol/mol
during the YD (Figure 2.3b) suggesting decreased SST. At first sight Mg/Ca ratios appear to
offer unambiguous records of SST, verifying findings by Lin et al (1997). However a second
species of G.ruber, pink variety has been analysed where white abundances are limited, and
there is no mention of whether Mg/Ca ratios are identical in both varieties. Conversely,
Herbert and Schuffert (2000) and Ruhleman et al (1999) report at the same time, alkenone
unsaturation indices (Uk37), another SST proxy, change only modestly (Figure 2.4), contrary
to findings by Lea et al (2003) of large SST changes. Uk37, similar to Mg/Ca ratios, are only
affected by water temperature. They measure the ratio of coccoliths with double carbon
bonds to triple bonds, with higher double bond ratios indicating lower temperatures (James,
2005). Limitations arise however as Uk37 typically records the most productive season, rather
than annual conditions.
Page 12 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Figure 2.4 – Mg/Ca derived SST of the Cariaco basin (Purple) (Lea et al., 2003) and alkenone
derived SST from the Caribbean Sea (green) (Ruhleman et al., 1999). Grey band indicates timing of
the YD period. Modified from Carlson (2013).
Lin et al (1997) have also examined the dominant primary producer within the surface
waters both just prior to and during the YD period. Prior to 12,600 ka the dominant
foraminiferal assemblage preserved in CB sediments was made up of G. ruber, a species
characterising non-upwelling seasons. At the onset of the YD, a shift in the dominant
foraminiferan species is seen. G. ruber gives way to and is almost wholly replaced by G.
bulloides, a species indicative of upwelling seasons and enhanced nutrient concentrations,
consistent with similar findings already mentioned by Peterson et al (1991) as a
consequence of the migration of the Inter Tropical Convergence Zone (ITCZ) and increased
upwelling. The works of Dahl et al (2004) also concur with both authors, in that dominant
primary production undergoes significant change at the YD onset. By utilising chlorine steryl
esters (CSEs), they report large scale changes in the phytoplankton community from
dinoflagellate dominated to diatom dominated during the YD. CSEs are a chemical
fingerprint of phytoplankton formed by estrification (a reaction between an alchohol and
acid) during zooplankton herbivory Dahl et al (2004). They show that CS1-2 dinoflagellate
falls during the YD, being replaced by CS3-4 diatom. These changes in primary production
mentioned appear to be universally agreed upon (Werne et al., 2000; Mertens et al., 2009),
with no conflicting views having been reported to date. Mertens et al (2009) report further
that dinoflagellate cysts decrease at the YD onset, with simultaneous reductions in Copepod
egg abundance, with both lines of evidence suggesting increased numbers of and predatory
action of dinoflagellates. There appears to be some limitation with the method proposed by
Page 13 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Dahl et al (2004) however, admitted by the author, that some sterols have been shown to
have similar molecular weight, and so species cannot always be reliably determined through
liquid chromatography mass spectrometry techniques. Further research into this method is
required in order to assess further its credibility, but the author is confident that this
technique would prove reliable in future instances of using CSEs as indicators of
phytoplankton populations and for past environmental reconstructions.
2.3. Timing the abrupt change.
With the various lines of evidence reported previously, placing a firm date the onset of the
YD is challenging. It has been attempted by Hughen et al (1996), through 20 accelerator
mass spectrometry 14C techniques on Globigerina bulloides. They propose that the timing of
the fall in greyscale values and increased radiocarbon occurred at 13 ka. Lea et al (2003) also
agree on the timing, by matching Mg/Ca ratios to greyscale values of multiple CB
sedimentary cores. The timing is not unanimously agreed upon by all authors and proxy
responses however. Lin et al (1997) and Dahl et al (2004) state that their proxy signals
record change beginning at 12.6 ka and 12.9 ka respectively, while Werne et al (2000) argue
that the onset of anoxia began also at 12.6 ka. At first sight, discrepancies of 400 years may
seem trivial on geological timescales, but considering the YD period itself lasted only around
1,000 years, this discrepancy represents a sizeable proportion of the period. In this respect,
ascertaining the most accurate dates possible for proxy responses is paramount to
understanding whether the YD onset was truly as rapid as reported, or whether it merely set
about a chain reaction of events over a more protracted period. In light of dating evidence
presented thus far, slower, more progressive chain of events seems the likely scenario.
3. Interpreting the evidence preserved in the sedimentary record.
3.1. Reconstructing the Younger Dryas environment of the Cariaco Basin.
Understanding proxy responses described in chapter 2 allows reconstruction of the CB
environment during the YD (Table 1). As mentioned, proxy responses are often affected by
multiple climatic variables, and interpreting their responses is sometimes subject to
conjecture. However, taken as the author’s results suggest; the oxygen isotope records
presented by Lin et al (1997) and the Mg/Ca ratios presented by Lea et al (2003) reveal that
at the YD onset, the CB underwent an abrupt transition to cooler conditions. Mg/Ca ratios
Page 14 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
suggest SST changes of 3-4°C according to T=(ln(Mg/Ca/0.38)/0.09)), whilst δ18O ratios
suggest changes of similar magnitude. Uk37 Indices presented by Ruhleman et al (1999)
paradoxically indicate warming of around 1°C over the region, according to Uk37 = 0.034T +
0.039. Thicker varves argued by Hughen et al (1996) suggest that the YD was also a period of
enhanced sedimentation as a consequence of increased primary production which, as
mentioned, similar to what is seen during today’s winter phases (Werne et al., 2000).
Reduced greyscale values argued by Dahl et al (2004) suggest further that the CB was much
more arid during the YD, with pronounced differences in wind direction and increased wind
strength as a result of the changing position of the ITCZ (Haug at al., 2001). The increases in
atmospheric 14C reported by Hughen et al (2000) suggest wholesale ocean circulation
reorganisation, specifically the thermohaline conveyor, which would have been either
slowed substantially, or stopped altogether, reducing poleward heat transport. The
abstraction of large amounts of seawater into the continental Arctic ice sheets as suggested
by Wilson et al (2007) would consequently lower sea levels around the CB by up to 120 m
Lin et al (1997), intensifying the isolation between it and the open Caribbean Ocean,
restricting water exchange to the almost immediate surface of the ocean. The switch from
dominantly dinoflagellate primary producers to diatom producers for the entirety of the
period (Dahl et al., 2004) attests to changes in upwelling strength and nutrient
concentration. During the YD, the CB must have been a site of strong, year round upwelling
in order to support such large primary producers such as diatoms.
Environmental variable Present Day Younger Dryas
SST 28°C 24°C
ITCZ location Seasonal Permanently displaced south
Wind direction/strength Seasonal Easterly - stronger
Aridity Low High
Thermohaline circulation Fully operational Limited or shut down
Sea level Average Up to 120 m lower
Ice caps Small Large
Primary production Flagellate dominated –
seasonal production
Diatom dominated – constant
high productionTable 1 – Tabular summary of the differences in the environment of the Cariaco Basin during the
Younger Dryas, compared to the present day. Compiled by Hughes, 2015.
Page 15 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
4. Processes and factors forcing abrupt climate change.
4.1. External forcing.
Some authors hypothesise that the YD was a consequence of an impact event (Alcantara et
al., 2012); citing nanodiamonds as evidence. They claim that nanodiamonds are formed by
immense and instantaneous pressures only achievable via massive impact events or large
detonations, pressures much greater than those which occur via natural geological
processes. van Hoesel et al (2014) however dispute this, reporting that despite the
nanodiamonds, there is a distinct lack of further geochemical evidence consistent with
impact events, along multiple timing discrepancies. However this naturally leaves the
question of what caused the nanodiamonds, if not an impact? Given that they do not occur
naturally on Earth. Perhaps there was indeed an impact, but not on the scale as to force the
abrupt cooling of the entire northern hemisphere. External forcing has been given brief
credit for the sake of completeness, but is not considered further.
4.2. Internal forcing, processes and feedback mechanisms.
Interpreting the complex interplay of feedback processes operating during the YD is difficult.
There is agreement between authors that reduced greyscale values represent increased
reflectivity caused by higher concentrations of light coloured plankton within the sediments
(Dahl et al., 2004; Hughen et al., 2000). Dahl and Hughen continue by claiming plankton
increases occurred due to rapid atmospheric reorganisation, and permanent (for the
duration of the YD) southward migration of the ITCZ (Figure 4.1); although the summertime
position of the ITCZ during the YD is uncertain at present (Riboulleau et al., 2014).
Page 16 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Figure 4.1 – Location of the present day ITCZ during summer periods, showing maximum rainfall
(a) and Location of the ITCZ during the YD, showing southerly displacement (b). From Riboulleau et
al (2014).
Southward migration brings the north east trade winds directly over the basin, inducing
Ekman upwelling, greatly increasing nutrient supplies and biological production (Lea et al.,
2003). Increases in upwelling intensity at the onset of the YD is consistent with further
claims by Dahl et al (2004) of a switch from dinoflagellate primary production to diatom
dominated production. Lalli and Parsons (1997) have shown in previous studies that an
influx of nutrient rich waters changes the size of the dominant primary producer, replacing
nanoplankton (flagellate) with macroplankton (diatom) as the former can no longer make
use of the larger, upwelling nutrients. The upwelling, increased primary productivity and
change to diatom dominated production is argued by Werne et al (2000) to be the cause of
the enhanced sedimentation that attempts to explain the low levels of OC through dilution.
Riboolleau et al (2011) however claim that the primary production change to diatoms lead
to a decrease of producers with organic carbon walls, instead diatoms exported mineral
tests. Despite both arguments being based on primary production shifts to diatoms, the
study by Riboulleau et al (2011) appears to offer a better explanation as to the cause of low
OC. These conflicts highlight a need for caution when using OC content of sediments as
palaeoenvironmental indicators. Both processes may also have acted in tandem, given the
substantially increased sedimentation of the period.
Haug et al (2001) argue that the cause of the southward migration was the 21 ka precession
cycle; by forcing less solar insolation to the northern hemisphere, the lower insolation
Page 17 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
would act to pull the ITCZ to the south. However in light of evidence presented next, and
claims by Wilson et al (2007) that the precession cycle is unlikely to be able to influence
global climate alone, it’s likely there was another dominant cause, with the precession cycle
merely acting along with it. In the works of Chiang and Bitz (2005), they claim the southward
displacement of the ITCZ was rather a consequence of the expansion of the polar ice caps.
They argue that the ITCZ shifts meridionally away from the hemisphere experiencing ice
sheet growth, which during the YD was the Northern (Wilson et al., 2007), causing
reorganisation of tropical precipitation and wind patterns and strengths, as the equator to
pole thermal gradient increases substantially.
Broecker (2003) argue that the increased ice sheet cover was a consequence of
thermohaline shutdown, giving credence to claims by Hughen et al (2000) that increases in 14C were caused by a reduction in North Atlantic Deep Water (NADW) formation, which
accounts for up to 75% of atmospheric 14C removal, transferring it to the deep ocean.
Muscheler et al (2000) also argue in favour of this; reporting that increased production via
increased solar activity suggested previously by Goslar and Arnold (2000) is unlikely as 10Be
flux measurements remains fairly constant throughout the period (Figure 4.2). The author
admits that changes in climate could affect the accumulation of 10Be; however the flux is
actually shown to be independent of climatic conditions recorded by oxygen isotopes. It is
also questionable as to whether the 10Be accumulating here is representative of that
accumulating globally, a similar argument against Goslar and Arnold’s lacustrine
measurements. Observations however suggest this is the case, as Greenland receives most
of its precipitation from lower latitudes, a pattern that has remained unchanging with time.
This 10Be record in my opinion forms a much more robust argument than that presented by
Goslar and Arnold (2000) and would appear to confirm that thermohaline perturbations
argued by Hughen et al (2000) as the cause of 14C increases.
Page 18 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Figure 4.2 - 10Be flux from the Greenland summit core. – Modified from (Muscheler et al., 2000).
Conversely, the conflicting increases in temperature are argued by Herbert and Schuffert
(2000) as a consequence of lower sea levels during the YD period, concomitant with ice cap
expansion reported by Chiang and Bitz (2005). They claim this acted to isolate the basin, and
allowed water exchange only between the warmer well mixed layers, which then provided a
temperature buffering effect. Wan et al (2009) add weight to the claim, arguing that during
glacial periods less heat is moved polewards during NADW shutdown, subsequently being
retained by the low latitudes, which ought to cause warming as recorded in Uk37 indices of
Ruhleman et al (1999). Both arguments can be considered credible, logical and consistent
with the observations of the evidence based on what we know so far. These conflicts then
perhaps represent a current gap in our knowledge and understanding of the processes and
mechanisms acting to force climatic change on small spatial scales during the YD.
5. Using the Cariaco Basin paleorecord to understand climate today.
5.1. What can we learn?
Understanding the behaviour of the past environment allows us to better understand the
climate today, by providing us with information that would otherwise take much longer
than human generations to obtain through present day observations. The CB palaeo-record
reveals that tropical regions are also susceptible to rapid climate change, just as are high
latitudes, whereas previously they were considered little affected (Wilson et al., 2007).
Effects are not as pronounced as in higher latitudes however and this may be why they have
been relatively overlooked in the research of rapid climate change. Effects are manifested as
complex temperature changes, and changes in the behaviour of coupled atmospheric/ocean
systems (Haug et al., 2001), which have the potential to induce wholesale changes in the
dominant primary producers in the area (Dahl et al., 2004). Through studying the CB
palaeorecord, we can begin to establish a picture of how the network of proxies responded
Page 19 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
to both one another, and also the forcing mechanisms acting to change the environment
during the period. Increasing our understanding of the past environment’s behaviour will
undoubtedly allow us to transfer this knowledge to the present day situation within the CB,
and allow us to understand today’s climate more thoroughly.
5.2. Implications for today’s climate.
In light of the literature presented so far, the CB of the YD appears to be a prolonged version
of today’s winter period (Werne et al., 2000), although with intrinsic differences in feedback
processes. Using our increased understanding gained from studying the CB during the YD
may give us a window into future climate change trajectories, especially prevalent given the
current concern surrounding anthropogenic CO2 emissions and the reduction in polar ice
volume. We have seen how a reduction in the thermohaline circulation can lead to rapid
glacial conditions throughout the northern hemisphere, leading to increased aridity and
changing wind and precipitation patterns. Current research claims that continued polar ice
melting could induce a freshwater “lid” over the NADW, and potentially shut it down
(Wilson et al., 2007). However, due to a lack of available literature, there is little research to
either confirm or refute this claim. Given we have seen the scenario that would potentially
follow this; the CB palaeorecord surely provides recourse for us to alter our future actions.
6. Discussion.
A less than satisfactorily clear picture of past climate change arises from the review of this
literature. Interpreting the proxies is a challenge, owing to their inherent nature of
recording multiple climatic variables, and focusing on one small locality may not provide a
representative or accurate example of average conditions during the YD. Each locality may
have different variables that can completely change the interpretation of the past
environment, which is not surprising, given only a very small selection of proxies here has
shown how complex the climate system can be.
Discrepancies often arise due to diagenesis, which could explain the Mg/Ca and Uk37
conflicts. Higher carbonate ion concentrations and anoxia argued by Lea et al (2003) and
Werne et al (2000) would appear to discount this possibility; however further information
on any water column degradation is lacking, and this could be significant. Claims of anoxia at
Page 20 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
12.6 ka with earlier greyscale reductions at 13 ka leaves ~400 years of oxic conditions for
potential diagenesis unaccounted for. If significant, this could blur and account for timing
discrepancies but not conflicting proxy signals, as they remain conflicting for the entire
period. However, given the extremely short timescale, geologically speaking, it is
questionable as to whether any significant diagenetic effects could manifest themselves in
this short time span.
It appears that the only universally agreed on process to have occurred within the CB during
the YD was that of the change in dominant primary production species. All authors have
agreed on the switch from flagellate to diatom dominated primary production, and this may
be so due to the limited variables that can affect the dominant primary producer. The
previous studies by Lalli and Parsons (1997) have shown that it is likely one variable that
drives primary producer size; the concentration of nutrients in the photic zone. Thus it may
be that for as long as there are multiple variables affecting proxy responses, controversy and
mixed interpretation will arise, prompting a need for further research.
Whether the timing discrepancies explained in chapter 2 are due to inaccuracies in age
determination through differing age calibration methods, or genuine lags in the proxies’
responses could be resolved by matching them to further proxies from locations known to
be affected worldwide. It may be that current dating methods for individual proxies are not
yet precise enough and subject to uncertainty that may improve in future studies and
methods.
Upwelling of cold, nutrient rich waters reported by Dahl et al (2004) could explain the
decreased SST claims of Lea et al (2003), however claims of G.ruber representing a reliable
annual proxy and not being associated with upwelling by the author appear to refute this.
Doubts presented on the methodology of using G.ruber due to its low abundance during the
period (Peterson et al., 1991) however call these claims into question. Also, there is no
mention of how ice cap expansion might have affected the δ18O values of Lin et al (1997),
nor any detail on how the salinity changes during the period have affected them. Perhaps it
may be that further research on the species’ seasonal behaviour today will provide further
insight.
Page 21 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Given its isolation to the wider Caribbean area, and sensitive position under the migrating
ITCZ (Riboulleau et al., 2014), the CB trend could represent a peculiarity to the larger region
and may explain the conflicting temperature signals between Lea et al (2003) and Ruhleman
et al (1999). This is especially prevalent given that Wan et al (2009) showed that during
NADW shutdown, the heat given up to warm the high latitude northern Atlantic would
naturally be retained in the tropics as the Gulf stream’s strength dwindles. It is difficult to
see where else, other than being retained by the tropics, this extra heat could go. Perhaps
the cessation of NADW formation would force the initiation of deep water formation
elsewhere, which is clearly not identifiable in the proxies of the CB. Further investigation
both to determine whether any migration of the ITCZ in YD summers occurred and whether
other tropical Atlantic, non upwelling areas showed cooling during the YD would be
prudent. This would go part of the way to answering questions in light of findings of
changing species calcifications depths by Tedesco et al (2007), and the warming and cooling
conflicts between Wan et al (2009) and Lea et al (2003).
These conflicting interpretations highlight needs for further study to increase our
understanding of abrupt tropical change. Perhaps multiple, multiproxy studies from other
regions of the tropical Atlantic, and perhaps further afield will be beneficial, in order to
generate a clearer picture of how the climate system behaved in the past.
7. Conclusion.
Signals of climate change are complex and must be treated with care, as despite a wealth of
information and research, controversy remains. Debates arise due to the close
teleconnections between high and low latitudes and the ocean/atmospheric system, acting
almost as one system, and separating out cause and effect is challenging.
Evidence of environmental change within the CB during the YD is abundant, ranging from
changing thicknesses and greyscale values of sedimentary laminations, increasingly positive
oxygen isotope ratios, increasingly negative Mg/Ca ratios, elevated 14C concentrations and
changes in foraminiferan assemblages. The evidence suggests that the CB underwent a
transition to cooler, more arid conditions, with altered upwelling, precipitation and wind
patterns. The dominant cause was found to be the southward displacement of the ITCZ for
Page 22 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
the duration of the period, which brought the trade winds directly over basin, concomitant
with expanding ice caps.
The CB palaeorecord is not without controversy however, and several proxy responses are
challenged by other authors. Current knowledge is based on time tested and robust
methodologies which lend strength to interpretations, however evidence of tropical climate
change is extremely limited geographically, compared to high latitudes, and so current
knowledge is based on a handful of locations only. These conflicts reveal that there are
currently gaps in our knowledge of rapid climate change during the YD in the tropical
Atlantic that further study could attempt to address. These studies should attempt to look at
the evidence of abrupt climate change within various localities of the tropical Atlantic,
Pacific and Indian oceans, along with establishing robust and precise dates of the responses
of climatic proxies. From this, we can ascertain a truly global picture of the climatic systems’
behaviour during the YD, and determine the processes and factors that were influencing the
behaviour of the environment. Once we are able to understand this, it will undoubtedly help
in our endeavours to understand the climate today, and future climate trajectories.
Evidence is much scarcer in low latitudes than in high latitudes however, and perhaps
existing methodologies also need re-visiting and improving in order to derive a higher
resolution picture of past tropical environments.
Throughout this literature review, my objectives have been well met. A wide range of
evidence has been presented, along with its timing, interpretation and the forcing processes
behind it, whilst always maintaining a critical and judgemental approach. Due to a lack of
available literature however, objective 5 is much less in depth than I would have hoped it
be.
Page 23 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
References
Alcántara, I., Bischoff, J., Vázquez, G., Li, H., DeCarli, P., Bunch, T., Wittke, J., Weaver, J., Firestone, R., West, A., Kennett, J., Mercer, C., Xie, S., Richman, E., Kinzie, C., Wolbach, W. (2012) 'Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis', Proceedings of the National Academy of Sciences of the United States of America, no. 13, pp. 4723 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edsjsr&AN=edsjsr.41588361&site=eds-live&scope=site (Accessed 19/05/2015). Broecker, W. (2003) 'Does the Trigger for Abrupt Climate Change Reside in the Ocean or in the Atmosphere?', Science, vol. 300, no. 5625, pp. 1519-1522 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edsjsr&AN=edsjsr.3834455&site=eds-live&scope=site (Accessed 29/04/2015).
Carlson, A.E. (2013) 'The Younger Dryas Climate Event', in Elias, S.A. (eds) Encyclopedia of Quaternary Science, Second Edition, Amsterdam, Elsevier [Online]. DOI:10.1016/B978-0-444-53643-3.00029-7 (Accessed 29/04/2015).
Chiang, J. and Bitz, C. (2005) 'Influence of high latitude ice cover on the marine Intertropical Convergence Zone', Climate Dynamics, vol. 25, no. 5, pp. 477-496 [Online]. DOI: 10.1007/s00382-005-0040-5 (Accessed 17/04/2015).
Clayton, T., Pearce, R., Peterson, L. (1999) 'Indirect climatic control of the clay mineral composition of Quaternary sediments from the Cariaco basin, northern Venezuela (ODP Site 1002)', Marine Geology, vol. 161, no. 2-4, pp. 191 [Online]. DOI: http://dx.doi.org/10.1016/S0025-3227(99)00036-5" (Accessed 15/05/2015)
Coe, A., Bosence, D., Church, K., Flint, S., Howell, J. and Wilson, R. (2005) The sedimentary record of sea-level change., Second edn. New York, Cambridge University Press.
Dahl, K., Repeta, D., Goericke, R. (2004) 'Reconstructing the phytoplankton community of the Cariaco Basin during the Younger Dryas cold event using chlorin steryl esters', Paleoceanography, vol. 19, no. 1, [Online]. DOI: 10.1029/2003PA000907 (Accessed 29/04/2015). Goslar, T. and Arnold, M. (2000) 'Variations of Younger Dryas atmospheric radiocarbon explicable without ocean circulation changes', Nature, vol. 403, no. 6772, pp. 877 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=a9h&AN=2869779&site=eds-live&scope=site (Accessed 22/05/2015). Haug, G., Hughen, K., Sigman, D., Peterson, L., Röhl, U. (2001) 'Southward Migration of the Intertropical Convergence Zone through the Holocene', Science, vol. 293, no. 5533, pp. 1304 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=a9h&AN=5123904&site=ehost-live&scope=site (Accessed 28/03/2015).
Herbert, T.D. and Schuffert, J.D. (2000) 'Alkenone unsaturation estimates of sea-surface temperatures at Site 1002 over a full glacial cycle', in Leckie, R.M., Sigurdsson, H., Acton, G.D., Draper, G. (Eds.) Proc. ODP, Sci. Results, 165, College Station, Texas (Ocean Drilling Program), 239–247. [Online]. doi:10.2973/odp.proc.sr.165.030.2000 (Accessed 04/04/2015).
Hughen, K., Overpeck, J., Peterson, L., Anderson, R. (1996) 'The nature of varved sedimentation in the Cariaco Basin, Venezuela, and its palaeoclimatic significance', Geological Society, London, Special Publications, vol. 116, no. 1, pp. 171-183 [Online]. DOI: 10.1144/GSL.SP.1996.116.01.15 (Accessed 29/04/2015).
Hughen, K., Southon, J., Lehman, S., Overpeck, J. (2000) 'Synchronous Radiocarbon and Climate Shifts during the Last Deglaciation', Science, vol. 290, no. 5498, pp. 09 March 2015 [Online]. Available
Page 24 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edsjsr&AN=edsjsr.3081660&site=eds-live&scope=site (Accessed 09/03/2015).
James, R. (2005) Marine Biogeochemical Cycles, Second edn. Milton Keynes, Open University.
Lalli, C. and Parsons, T. (1997) Biological Oceanography: An introduction, Second edn. Oxford, Butterworth-Heinemann.
Lea, D., Pak, D., Peterson, L., Hughen, K. (2003) 'Synchroneity of Tropical and High-Latitude Atlantic Temperatures over the Last Glacial Termination', Science, vol. 301, no. 5638, pp. 09 March 2015-1361-1364 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edsjsr&AN=edsjsr.3835024&site=eds-live&scope=site (Accessed 09/03/2015)
Lin, H., Peterson, L.C., Overpeck, J.T., Trumbore, S.E., Murray, D.W. (1997) 'Late Quaternary climate change from δ18O records of multiple species of planktonic foraminifera: High-resolution records from the Anoxic Cariaco Basin, Venezuela', Paleoceanography, vol. 12, no. 3, pp. 415-427 [Online]. DOI: 10.1029/97PA00230 (Accessed 28/04/2015)
Mertens, K., Gonzalez, C., Delusina, I., Louwye, S. (2009) '30 000 years of productivity and salinity variations in the late Quaternary Cariaco Basin revealed by dinoflagellate cysts', Boreas, vol. 38, no. 4, pp. 647-662 [Online]. DOI: 10.1111/j.1502-3885.2009.00095.x (Accessed 02/05/2015).
Muscheler, R., Beer, J., Wagner, G., Finkel, R. (2000) 'Changes in deep-water formation during the Younger Dryas event inferred from 10Be and 14C records', Nature, vol. 408, no. 6812, pp. 567-570 [Online]. DOI: doi:10.1038/35046041 (Accessed 21/04/2015).
Peterson, L.C., Overpeck, J.T., Kipp, N.G. and Imbrie, J. (1991) 'A High-Resolution Late Quaternary Upwelling Record from the Anoxic Cariaco Basin, Venezuela', Paleoceanography, vol. 6, no. 1, pp. 99-119 [Online]. DOI: 10.1029/90PA02497 (Accessed 29/04/2015).
Riboulleau, A., Tribovillard, N., Baudin, F., Bout-Roumazeilles, V., Lyons, T. (2011) 'Unexpectedly low organic matter content in Cariaco Basin sediments during the Younger Dryas: Origin and implications', Comptes Rendus Geoscience, vol. 343, no. 5, pp. 351 [Online]. DOI: http://dx.doi.org/10.1016/j.crte.2011.04.001" (Accessed 15/03/2015).Riboulleau, A., Bout-Roumazeilles, V. and Tribovillard, N. (2014) 'Controls on detrital sedimentation in the Cariaco Basin during the last climatic cycle: insight from clay minerals', Quaternary Science Reviews, vol. 94, no. 0, pp. 62 [Online]. DOI: http://dx.doi.org/10.1016/j.quascirev.2014.04.023" (Accessed 30/04/2015).Ruhlemann, C., Mulitza, S., Muller, P., Wefer, G., Zahn, R. (1999) 'Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation', Nature, vol. 402, no. 6761, pp. 511-514 [Online]. DOI:10.1038/990069 (Accessed 21/04/2015).
Tedesco, K., Thunell, R., Astor, Y., Muller-Karger, F. (2007) 'The oxygen isotope composition of planktonic foraminifera from the Cariaco Basin, Venezuela: Seasonal and interannual variations', Marine Micropaleontology, vol. 62, no. 3, pp. 180-193 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edswsc&AN=000244392700003&site=eds-live&scope=site (Accessed 14/04/2015). van Hoesel, A., Hoek, W., Pennock, G. and Drury, M. (2014) 'The Younger Dryas impact hypothesis: a critical review', QUATERNARY SCIENCE REVIEWS, vol. 83, pp. 95-114 [Online]. Available at http://libezproxy.open.ac.uk/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=edswsc&AN=000331673400009&site=eds-live&scope=site (Accessed 19/05/2015). Werne, J., Hollander, D., Lyons, T., Peterson, L. (2000) 'Climate-induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela', Paleoceanography, vol. 15, no. 1, pp. 19-29 [Online]. DOI: 10.1029/1998PA000354 (Accessed 15/03/2015).
Page 25 of 26
Lewis Hughes - B8470472 – SXG390 – EMA
Wilson, R., Hyden, F., Coe, A. (2007) The Great Ice Age, 2nd edn. Cambridge, The Open University.
Wan, X., Chang, P., Saravanan, R., Zhang, R. and Schmidt, W. (2009) 'On the interpretation of Caribbean paleo-temperature reconstructions during the Younger Dryas', Geophysical Research Letters, vol. 36, no. 2, [Online]. DOI: 10.1029/2008GL035805 (Accessed 29/04/2015
Page 26 of 26