Cariaco Basin - Literature Review

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

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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)

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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

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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

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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.

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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).

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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.

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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.

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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)

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(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

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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.

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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.

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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

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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

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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.

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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).

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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

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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.

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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

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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


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.

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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


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.

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Cariaco Basin - Literature Review