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Sachs J.P., Pahnke K., Smittenberg R. and Zhang Z. (2013) Biomarker Indicators of Past Climate. In: Elias S.A. (ed.) The Encyclopedia of Quaternary Science, vol. 2, pp. 775-782. Amsterdam:

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Biomarker Indicators of Past ClimateJ P Sachs, University of Washington, Seattle, WA, USAK Pahnke, University of Oldenburg, Oldenburg, GermanyR Smittenberg, Stockholm University, Stockholm, SwedenZ Zhang, Nanjing University, Nanjing, People’s Republic of China

ã 2013 Elsevier B.V. All rights reserved.

Introduction

Molecular fossils, ‘biomarkers,’ add to the growing arsenal of

climate indicators scientists are using to reconstruct climate

history and understand what causes climate to change. Ad-

vances in analytical chemistry and instrumentation over the

last two decades have expanded the analytical window in

which organic geochemists can routinely work. Precise and

highly sensitive mass spectrometers that can determine molec-

ular structures and isotope ratios are now coupled to gas and

liquid chromatographs that can separate the complex mixtures

of organic constituents found in geologic samples into pure

components.

As more geoscientists become familiar with these new tech-

niques, there are likely to be tremendous advances in organic

geochemical climate proxies. This article reviews some of the

more promising and novel biomarker proxies now being used

in paleoclimate research. The article is organized around the

climate parameters of most interest to paleoceanographers and

paleoclimatologists: temperature, salinity, and sea ice.

Biomarker Proxies of Temperature

The reconstruction of ocean and lake surface temperatures is

central to understanding past climatic changes. Impressive

advances have been made in organic geochemical approaches

to temperature reconstructions. This article focuses on the two

most promising and widely applied organic geochemical water

temperature proxies: Uk037 and TEX86. A relatively new organic

geochemical indicator of air temperature known as cyclization

ratio of branched tetraethers/methylation index of branched

tetraethers (CBT/MBT) is also briefly discussed.

Alkenone Paleothermometry

The discovery of temperature-controlled unsaturation patterns

in long-chain ketones and alkenones (nC37–nC39 methyl and

ethyl ketones) in the mid-1980s (Brassell et al, 1986), and their

identification as biomolecules unique to prymnesiophyte algae,

principally the coccolithophorid Emiliania huxleyi (Marlowe

et al., 1984; Volkman et al., 1980), was a watershed event in

paleoclimate research. For the first time paleoceanographers had

a tool for reconstructing sea surface temperatures (SST) that was

unambiguously derived from organisms living in the uppermost

part of the ocean where sunlight can penetrate, that was not

influenced by the chemical composition of seawater (as, for

instance, oxygen isotope ratios in planktonic Foraminifera

are), and that was not corrupted by the chemical composition

of ocean bottom water. Thus, a sizeable advantage of the

Encyclopedia of Quaternary Scien

alkenone paleotemperature technique over other temperature

proxies, such as Mg/Ca ratios in planktonic Foraminifera or

faunal (Foraminifera, radiolarians, diatoms) assemblage

changes, is the fact that alkenones are preserved in the sediments

even after the dissolution of the calcareous remains of their

producers. The technique can, therefore, be applied in regions

where other techniques would fail (e.g., the Southern Ocean).

The original definition of the unsaturation ratio (Uk37

index) included the tetraunsaturated C37 alkenone (Brassell

et al, 1986), which was later omitted due to its insignificant

contribution (Uk037¼C37:2/(C37:2þC37:3)). Prahl et al. (1988)

were the first to publish a quantitative calibration of the Uk037-

index to growth temperature (Uk037¼0.034 Tþ0.039,

r2¼0.994) based on cultured E. huxleyi, and initial field mea-

surements showed that this calibration reproduced SSTs in the

northeast Pacific Ocean (Prahl and Wakeham, 1987). This

calibration has since been widely used for the reconstruction

of past SSTs inmost parts of the world’s oceans (e.g., Bard et al.,

1997; Herbert et al., 2001; Koutavas and Sachs, 2008; Pahnke

and Sachs, 2006; Sachs and Lehman, 1999). Extensive alke-

none analyses on a global set of core-top samples and correla-

tion of the Uk037 index with overlying mean annual SSTs

(Uk037¼0.033 Tþ0.069, r2¼0.981) have further confirmed

the culture-derived relation of Prahl et al. (1988) (Muller

et al., 1998) and demonstrated its applicability to most oceanic

areas (Figure 1). Though slight differences exist in the Uk037-

temperature dependence of different coccolithophorid species

and genetic variations (Conte et al., 1998), past changes in

species composition do not appear to compromise paleo-SST

reconstructions (Muller et al., 1997).

Most studies, including the core-top Uk037-SST calibration

of Muller et al. (1998), assume that Uk037 values reflect tem-

peratures at the sea surface. However, alkenone concentrations

in sediment traps indicate that the depth of maximum produc-

tion varies regionally and can even change between different

seasons. Alkenone-producing haptophytes show highest abun-

dance in the surface mixed layer in the North Atlantic (Conte

et al., 2001) and in the northwest Pacific (Hamanaka et al.,

2000). Similarly, Ohkouchi et al. (1999) concluded from a

sediment transect in the Pacific that alkenones are produced

in the surface mixed layer at high and low latitudes, while

thermocline production prevails at Mid-Latitudes. In the gyre

region of the subtropical North Pacific, haptophyte growth

is linked to the depth of the chlorophyll maximum, which

can vary in depth throughout the year (Prahl et al., 2005).

Alkenone production shows a strong seasonal cycle in most

areas (Conte et al., 2001; Leduc et al., 2010; Prahl et al., 2005),

but the unsaturation ratio preserved in the sediments predom-

inantly reflects mean annual temperatures (Muller and Fischer,

2001; Prahl et al., 1993, 2005). In contrast, a strong summer

775

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776 PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Biomarker Indicators of Past Climate

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to fall bias in Uk037-SST estimates is observed in cold, high-

latitude regions of the ocean (Prahl et al., 2010; Sikes and

Volkman, 1993; Sikes et al., 2005; Ternois et al., 1998), sug-

gesting that in some areas, regional Uk037-SST calibrations bet-

ter reproduce SSTs.

Alkenones have been found in sediments as old as the early

Cretaceous (Brassell et al., 2004) and they have been used to

reconstruct SSTs on time scales ranging from 10–106 years.

Global core-top calibration (60° S-60° N)

Estim. SST = (UK’37−0.044)/0.033

30

30

25

25

20

20

Est

imat

ed S

ST

(°C

)

15

15

10

10

Annual mean SST, 0 m (°C)

5

50

0

Figure 1 Global core-top calibration of Uk037-derived SST versus

overlying mean annual SST from Muller et al. (1998)(SST¼Uk0

37�0.044)/0.033, r 2¼0.958).

Retention time

I

I

(a)

(b)Rel

ativ

e in

tens

ity

II

II

III

III

IV

V

VI

VI

V

V’

V’

VII

VII

VIII

HO

HO

HO

OH

HO

HO

HOVII

VII

VIb

OH

OH

VIII

Figure 2 Base peak HPLC-MS chromatograms of GDGTs with the various strTEX86 value of 0.42 and a BIT index value of around 0.6. (b) Sample from theReproduced from Schouten S, et al. (2009) An interlaboratory study of TEX86spectrometry. Geochemistry, Geophysics, Geosystems 10: Q03012.

Encyclopedia of Quaternary Scienc

Despite substantial alkenone loss in both the water column

and the sediment after deposition, several studies indicate

stability of the Uk037-index over geologic time and, therefore,

no effect on paleo-SST reconstructions (Conte et al., 1992;

Muller and Fischer, 2001; Prahl et al., 1989, 1993). However,

contrasting studies indicate differential removal of C37:3 (Gong

and Hollander, 1999; Rontani et al., 2005, 2008), which can

have implications for alkenone paleothermometry, depending

on the conditions under which degradation occurred (e.g.,

anoxic, oxic, denitrifying). Other environmental factors poten-

tially affecting the unsaturation ratio of alkenones include light

limitation (causing a bias towards warmer SSTs) and nutrient

limitation (bias towards lower SSTs; Prahl et al., 2003).

The TEX86 and MBT/CBT Paleotemperature Proxies

Over the last decade, two new paleotemperature proxies were

developed based upon the relative abundances of various glyc-

erol dialkyl glycerol tetraethers (GDGTs) (Figure 2). Isoprenoi-

dal GDGTs with sn-2,3 stereochemistry are produced by at least

two of three main groups of archaea, the Crenarchaeota and the

Euryarchaeota (De Rosa and Gambacorta, 1988; Koga and

Morii, 2005), that occur ubiquitously in the environment,

including in the open ocean (e.g., Karner et al., 2001), lakes

(e.g., Powers et al., 2010), and soils (Leininger et al., 2006).

Crenarchaeota biosynthesize different types of GDGTs contain-

ing 0–3 cyclopentane moieties (I–IV; Figure 2) and crenarch-

aeol (V), which, in addition to four cyclopentane moieties, has

a cyclohexane ring (Schouten et al., 2000; Sinninghe Damste

I

II

III

IV

V

HO

HO

HO

HO

HOOH

HO

HO

HO

OH OH

OH

OH

OH

OH

Ib

b

VIc

VIIc

VIIIc

OH

OH

OH

OH

uctures indicated. (a) Sample from the Norwegian Drammensfjord, with aArabian Sea, with a TEX86 value of 0.68 and a BIT index of <0.05.and BIT analysis using high-performance liquid chromatography-mass

e, (2013), vol. 2, pp. 775-782

PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Biomarker Indicators of Past Climate 777

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et al., 2002). They also synthesize small quantities of a regioi-

somer of crenarchaeol (V0).Other GDGTs have an sn-1,2 stereochemistry and n-alkyl

chain architecture, and contain varying amounts of methyl

branches and cyclopentane moieties (VI–VIII, Figure 2). The

exact biological source of these so-called branched GDGTs re-

mains uncertain. Initially attributed to soil bacteria (Herfort

et al., 2006; Hopmans et al., 2004; Sinninghe Damste et al.,

2000; Weijers et al., 2006), they can also be produced, as

suggested by recent evidence, in sediments or in the water

column (Peterse et al., 2009a,b; Sinninghe Damste et al.,

2009; Tierney and Russell, 2009).

Culture studies have shown that the relative number of cyclo-

pentanemoieties in theGDGTs fromhyperthermophilic archaea

increases with growth temperature (Gliozzi et al., 1983) (Uda

et al., 2001), which has been interpreted as a means by which

archaea adjust their cell membrane rigidity. This dependency

on growth temperature was also demonstrated for the nonhy-

perthermophilic achaea (Wuchter et al., 2004). Thus, by mea-

suring the relative amounts of GDGTs present in marine

sediments, the temperature at which Crenarchaeota were living

when they produced their membranes can be determined. The

TEX86 proxy was proposed as a means to quantify the relative

abundance of GDGTs (Schouten et al., 2002):

TEX86 ¼ III½ � þ IV½ � þ V0� �

II½ � þ III½ � þ IV½ � þ V0� � [1]

The roman numerals refer to structures in Figure 2. The

original 44 sediment core-tops from 15 different locations used

to establish this new paleotemperature proxy were expanded in

the years that followed. Kim et al. (2008) analyzed 287 core-

top sediments distributed over the world oceans from different

depths, and after exclusion of samples from the Red Sea and

the polar oceans, as well as another fifteen outliers, they arrived

at the following global temperature calibration:

SST ¼ �10:78þ 56:2� TEX86 r2 ¼ 0:935; n ¼ 223� �

[2]

Two years later, Kim et al. (2010) expanded this dataset

again to 426 core-tops, and proposed a new calibration:

SST ¼ 67:5� TEXL86

þ 46:9 r2 ¼ 0:86; n ¼ 396, p < 0:0001� �

[3]

with

TEXL86 ¼ log

III½ �II½ � þ III½ � þ IV½ �

� �[4]

Besides being logarithmic, this new calibration excludes the

regioisomer of crenarchaeol (structure V0). It appears better

suited to colder temperatures (<15 �C) including those of the

polar oceans. For temperatures >15 �C, Kim et al. (2010)

proposed a logarithmic calibration based on the original

TEX86 proxy:

SST ¼ 68:4� log TEX86

þ 38:6 r2 ¼ 0:87;n ¼ 255; p < 0:0001� �

[5]

The TEX86 temperature proxy has also been shown to work

in large lakes characterized by high Crenarchaeotal production

(Powers et al., 2010). They proposed the following modified

TEX86 calibration specifically for lakes:

Encyclopedia of Quaternary Scien

LST ¼ �14:0� TEX86 þ 55:2 [6]

This calibration varies only slightly from the ‘original’ ma-

rine calibration (eqn [2]), suggesting that salinity does not have

a large effect on the TEX86 proxy, a conclusion also reached by

Wuchter et al. (2004) based on mesocosm experiments.

The TEX86 SST proxy has been primarily applied in open

ocean studies, where the confounding influence of terrestrially

sourced GDGTs is minimized. Examples are late Quaternary

water temperature reconstructions from the Arabian Sea

(Huguet et al., 2006b), the African lakes (Tierney et al., 2008),

the Mediterranean Sea (Castaneda et al., 2010), the Eocene

Arctic Ocean (Brinkhuis et al., 2006; Sluijs et al., 2006), and

the Cretaceous proto-Atlantic (Schouten et al., 2003).

Several limitations and potential confounding influences

must be considered when applying and interpreting TEX86-

based temperature reconstructions. Several studies (Wakeham

et al., 2003; Wuchter et al., 2005, 2006) have shown that, even

though achaea live throughout the water column and hence

produce GDGTs at a variety of depths (Herndl et al., 2005;

Ingalls et al., 2006; Karner et al., 2001), sedimentary TEX86

values typically reflect the SST of water layers <100 m deep.

This is likely due to the efficient export of organic matter from

the surface ocean in the form of aggregates and zooplankton

fecal pellets (Huguet et al., 2006a). This is, however, not always

the case, and sedimentary GDGTs can reflect the average tem-

perature of the entire water column (Menzel et al., 2006).

Another potential source of uncertainty is the seasonally vary-

ing production of GDGTs (e.g. Leider et al., 2010; Sinninghe

Damste et al., 2009; Wuchter et al., 2005) though sediment

core-top TEX86 values do appear to reflect mean annual tem-

peratures (Kim et al., 2008). This is likely the result of season-

ally invariant archaeal growth and export. Oxidation of GDGTs

under prolonged oxic conditions, such as can occur in turbi-

dites deposited in the deep ocean, can also lead to changes in

TEX86 values equivalent to 2–6 �C (Huguet et al., 2009). The

relative abundance of Crenarchaeota versus Euryarchaeota or

even different Crenarchaeotal populations that may produce

different relative abundances of GDGTs can also influence

TEX86-based SSTs (Schouten et al., 2008; Turich et al., 2007,

2008). This is particularly likely in coastal, lacustrine, or iso-

lated basins (Trommer et al., 2009) as well as in upwelling

areas (Lee et al., 2008), where certain strains of Crenarchaeota

and Euryarchaeota might be favored. In anoxic settings, such as

highly organic-rich sediments and anoxic basins, methano-

trophic achaea may also be a significant source of GDGTs to

sediments (Blaga et al., 2009; Sinninghe Damste et al., 2009;

Wakeham et al., 2004).

Besides the branched GDGTs, soil organic matter also typ-

ically contains some of the isoprenoidal GDGTs I, II, and III

and some crenarchaeol. A significant contribution of these

GDGTs prohibits the use of the TEX86 proxy; this can be

evaluated using the branched versus isoprenoid tetraether

(BIT) index:

BIT ¼ VI½ � þ VII½ � þ VIII½ �V½ � þ VI½ � þ VII½ � þ VIII½ � [7]

Values above 0.3 may be used as a general limit, although

evaluation of the terrestrial andmarine end-members is recom-

mended when possible (Weijers et al., 2006). Because of this

ce, (2013), vol. 2, pp. 775-782

778 PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Biomarker Indicators of Past Climate

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complication, the TEX86 cannot be used in many coastal

settings and in most lakes (Blaga et al., 2009; Powers et al.,

2004, 2010).

The distribution patterns of branched GDGTs have also

been explored for their utility as paleoenvironmental proxies.

The relative amount of cyclopentane moieties (VI–VIIIb and c,

Figure 2), expressed as the CBT index, was found to be related

to the pH of the soil, while the relative amount of methyl

branches, expressed as the MBT, was positively correlated

with the annual mean air temperature and, to a lesser extent,

to soil pH (Peterse et al., 2009b; Weijers et al., 2007).

CBT ¼ � logVIIb½ � þ VIIIb½ �VII½ � þ VIII½ �

� �[8]

MBT¼VIII½ �þ VIIIa½ �þ VIIIb½ �

VI½ �þ VII½ �þ VIII½ �þ VIa½ �þ VIIa½ �þ VIIIa½ �þ VIb½ �þ VIIb½ �þ VIIIb½ �[9]

CBT¼3:33�0:38�pH r2¼0:70; n¼134� �

[10]

MBT¼0:867�0:096�pHþ0:021

�MAT r2¼0:82; n¼134� �

[11]

The latter two equations can be combined:

MBT ¼ 0:122� 0:187� CBT þ 0:020

�MAT r2 ¼ 0:77� �

[12]

One example of the use of this proxy is the reconstruction

of East Asian continental temperatures during the last deglaci-

ation period (Peterse et al., 2010).

Biomarker Proxies of Salinity

Along with temperature, salinity is the most important oceano-

graphic parameter. Its reconstruction can shed light on changes

in ocean stratification, water mass circulation, and precipitation,

which are tightly linked to climate. Recent developments in

organic geochemistry have produced two promising paleosali-

nity proxies: the relative abundance of tetraunsaturated C37

methyl ketones (alkenones) and the hydrogen isotopic compo-

sition (D/H)of organic compounds producedbyphytoplankton

and plants.

The %C37:4 Paleosalinity Proxy

The C37:4 alkenone is produced primarily in cold surface waters

of the ocean (temperatures <10 �C) (Rosell-Mele et al., 1994)

and is not well preserved in sediments compared to the di- and

triunsaturated alkenones. It is, therefore, most often excluded

from alkenone paleotemperature reconstructions (see the

Table 1 %C37:4-Salinity relationships in different ocean basins

Ocean basin Linear regression

North Atlantic and Nordic Seas %C37:4¼146.9(�10.6)�4.1S (North Atlantic %C37:4¼48.1Sþ1691Bering Strait %C37:4¼11.7Sþ397.6

Encyclopedia of Quaternary Scienc

section ‘Alkenone Paleothermometry’). In polar regions, how-

ever, the inclusion of C37:4 into the unsaturation ratio (Uk37)

can, in some cases, increase its correlation with SST (Bendle

and Rosell-Mele, 2004).

At concentrations of %C37:4 (relative abundance of C37:4 to

the total abundance of C37:2, C37:3, and C37:4, as percentage)

>5%, the temperature relation breaks down (Rosell-Mele,

1998; Sicre et al., 2002; Sikes et al., 1997), however, and a

correlation with salinity has, instead, been suggested in the

Nordic Seas and the North Atlantic (Rosell-Mele, 1998;

Rosell-Mele et al., 2002; Sicre et al., 2002; Table 1). A %C37:4-

salinity dependence was also observed in the low-salinity wa-

ters of the Sea of Okhotsk (Seki, 2005), the Bering Strait

(Harada et al., 2003), and the Baltic Sea (Blanz et al., 2005;

Schulz et al., 2000; Table 1), leading to the suggestion that low

salinities may impose stress on the alkenone-producing hap-

tophytes, causing them to increase production of C37:4 (Harada

et al., 2003).

The established %C37:4-salinity relations, however, vary sig-

nificantly between oceanic regions (Table 1), and no correla-

tion with salinity was found in particulate and surface

sediment samples from the Nordic Seas (Bendle et al., 2005)

or the Atlantic, Pacific, and Indian sectors of the Southern

Ocean (Sikes and Sicre, 2002). The applicability of %C37:4 as

a salinity proxy may thus be restricted to certain cold, low-

salinity marine environments. Further studies will need to

establish whether this is due to a possible contribution to

C37:4 production from other environmental factors. Neverthe-

less, the importance of establishing past salinity variations

justifies this further work.

The Lipid dD Paleosalinity Proxy

The hydrogen isotopic composition (dD¼ [(D/H)sample/

(D/H)standard�1]�1000), where the standard is Vienna Stan-

dard Mean Ocean Water of lipids from plants and algae reflect

the dD value of environmental water (Englebrecht and Sachs,

2005; Sachse et al., 2004; Sauer et al., 2001; Sessions et al.,

1999; Zhang and Sachs, 2007). Precipitation dD values, in

turn, reflect air temperature, air mass trajectory, intensity of

precipitation, and the isotopic composition of source water.

Temperature tends to dominate at high latitudes, while precip-

itation intensity tends to dominate in the tropics, such that

lower precipitation dD values occur at lower temperatures and

higher precipitation intensities (Dansgaard, 1964). Since most

hydrogen in lipids is bound to carbon and, therefore, nonex-

changeable on time scales of millions of years, the dD value of

sedimentary lipids contains important information on past

hydrological conditions and climate.

dD values in alkenones have been shown in culture and

field studies to closely record the isotopic composition of the

R2 Reference

�0.3) 0.69 Rosell-Mele et al. (2002), p. 13010.78 Sicre et al. (2002), p. 13040.76 Harada et al. (2003), p. 1310

e, (2013), vol. 2, pp. 775-782

PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Biomarker Indicators of Past Climate 779

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water in which they were produced (Englebrecht and Sachs,

2005). Alkenone dD values are depleted relative to that of the

water by some �193% (Englebrecht and Sachs, 2005), reflect-

ing the initial deuterium depletion of the primary photosyn-

thate of �171% (Yakir and DeNiro, 1990) and further

fractionation during alkenone biosynthesis. A greater D deple-

tion of C37:3 relative to C37:2 requires chromatographic separa-

tion of the individual alkenones prior to isotope analysis

(D’Andrea et al., 2007; Schwab and Sachs, 2009). Moreover,

different hydrogen isotope fractionation by the two most abun-

dant producers of alkenones in the modern ocean, E. huxleyi

and Gephyrocapsa oceanica (dD offset �30%; Schouten et al.,

2006), requires knowledge of the species distribution and their

temporal changes for robust paleoceanographic reconstructions

based upon alkenone dD values.

D/H fractionation during the biosynthesis of alkenones and

a wide variety of other phytoplanktonic and cyanobacterial

lipids has been recently shown to occur in response to salinity

changes (Schouten et al., 2006; Sachse and Sachs, 2008; Sachs

and Schwab, 2011). D/H fractionation decreased by an average

of 0.9� 0.2% per unit (�ppt) increase in salinity in a variety of

lipids from hypersaline ponds on Christmas Island in the

tropical Pacific Ocean (Sachse and Sachs, 2008) and in the

dinoflagellate sterol dinosterol from the Chesapeake Bay estu-

ary in the eastern United States (Sachs and Schwab, 2011)

(Figure 3). This response to salinity is most often additive to

the effect of the dD value of the environmental water, in which

arid conditions result in higher evaporation relative to precip-

itation, and, therefore, higher water dD values and vice versa.

This increases the sensitivity of biomarker dD as a salinity

proxy for the quantitative reconstruction of past changes in

the hydrological cycle.

The lipid dD paleosalinity proxy has been applied to recon-

struct salinity variations in late Quaternary ocean sediments

(Pahnke et al., 2007; van der Meer et al., 2007) and from late

100

Slope = 0.9 (± 0.2) ‰/PS

aLi

pid

-wat

er

50

Salinity

00.65

0.7

0.75

0.8

0.85

0.9

0.95

Figure 3 Fractionation factors for a variety of lipids from hypersaline pondsBay estuary (Sachs and Schwab, 2011). The average slope of all regressionsfractionation of 0.9% per unit increase in salinity.

Encyclopedia of Quaternary Scien

Holocene sediments in the Black Sea (van der Meer et al.,

2008) and from saline lakes on tropical Pacific islands (Sachs

et al., 2009; Smittenberg et al., 2011).

There is still much we do not understand about the envi-

ronmental influences on D/H fractionation during lipid syn-

thesis in phytoplankton and cyanobacteria. In addition to

salinity, recent work indicates that growth rate, growth phase,

and growth temperature, all seem to influence D/H fraction-

ation in phytoplankton (Wolhowe et al., 2009; Zhang et al.,

2009). But these effects may be small in comparison to the

water dD and salinity effect in many settings.

The Highly Branched Isoprenoid Sea Ice Proxy (IP25)

C25 and C30 highly branched isoprenoid (HBI) alkenes are

ubiquitous components in recent sediments that appear to

originate from four genera of marine diatoms (Class Bacillar-

iphyceae), Rhizosolenia, Haslea, Navicula, and Pleurosigma

(Grossi et al., 2004). Several C25 HBI alkenes (C25:1 to C25:6)

have been characterized in cultures of Haslea and Pleurosigma

(Wraige et al., 1997).

The reported covariance of numbers of unsaturations in

HBI alkenes in diatoms with temperature once represented a

hopeful new paleotemperature proxy (Rowland et al., 2001).

However, this proxy has not been successfully applied to paleo-

temperature reconstructions.

Recently, a unique C25 monounsaturated HBI has been

detected in sea ice samples. Some Haslea species have been

reported to inhabit Arctic sea ice (Booth and Horner, 1998).

The recent sediments from the Arctic with this C25 monoun-

saturated HBI underlie either seasonal or permanent ice cover,

and it has yet to be reported in sediments frommore temperate

environments or from northern high-latitude open-water or

ice-free phytoplankton (Belt et al., 2007). As a result, the

U

150y = 0.68 + 0.00099x R2= 0.57

y = 0.78 + 0.00076x R2= 0.66

y = 0.72 + 0.00082x R2= 1

y = 0.65 + 0.0011x R2= 0.91

y = 0.81 + 0.00069x R2= 0.85

y = 0.77 + 0.00074x R2= 0.48

y = 0.81 + 0.0008x R2= 0.79

CB dinosterolXmas TLEXmas DiplopteneXmas PhyteneXmas nC18:1Xmas nC18Xmas nC17:1Xmas nC17

y = 0.76 + 0.0012x R2= 0.95

on Christmas Island (Sachse and Sachs, 2008) and the Chesapeakeis 0.0009�0.0002, implying a relatively constant decrease in D/H

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monounsaturated isomer has been attributed to sea ice dia-

toms, and its existence in sediments is an indicator of sea ice.

The proxy has been named IP25 (ice proxy with 25 carbons)

(Belt et al., 2007).

The IP25 records from sediments have proven to be remark-

ably consistent with historical sea ice records and with other

paleoclimatic parameters (Gregory et al., 2010; Masse et al.,

2008). The d13C values of IP25 in sea ice and in sediment trap

material collected under sea ice are also distinguishably heavier

than that of organic matter from planktonic or terrigenous

sources, supporting the use of IP25 as an Arctic sea ice proxy

(Belt et al., 2008).

IP25 has been applied for the reconstruction of sea ice over

the last several centuries (Vare et al., 2010) and during the last

30000 years (Muller et al., 2009).

Summary

Tremendous progress has beenmade in the last decade develop-

ing and applying organic geochemical indicators of past climate.

A small set of the most significant of these advances has been

reviewed here. Advances in analytical chemistry in the coming

years ought to make all of the temperature and hydrologic prox-

ies described here available to a broad range of geochemists. The

different biological, physical, and chemical underpinnings of

these proxies, as compared to the inorganic paleoclimate proxies

widely used today, such as the oxygen and carbon isotopes and

trace metal ratios in foraminiferal shells, make them comple-

mentary. In some climatic or depositional environments, one or

another of the temperature or hydrologic proxies may work

best, while in other settings it ought to be possible to apply two

ormore indicators of the same climate parameter within a single

sediment core. As a result, increasingly robust reconstructions

of climate are certain to emerge.

See also: Diatom Introduction; Structures and Applications ofBiomarkers from Arctic Sea Ice Diatoms. Carbonate StableIsotopes: Lake Sediments. Diatom Methods: d18O Records;Diatomites: Their Formation, Distribution, and Uses; Salinity andClimate Reconstructions from Continental Lakes. Paleobotany:Ancient Plant DNA; Overview of Terrestrial Pollen Data.Paleoceanography, Biological Proxies: Coccolithophores;Dinoflagellates; Marine Diatoms; Planktic Foraminifera; Radiolariansand Silicoflagellates; Temperature Proxies, Census Counts.Paleoceanography, Physical and Chemical Proxies: CarbonCycle Proxies (d11B, d13Ccalcite, d

13Corganic, Shell Weights, B/Ca, U/Ca,Zn/Ca, Ba/Ca); Mg/Ca and Sr/Ca Paleothermometry from CalcareousMarine Fossils; Nutrient Proxies. Paleoclimate Reconstruction:Approaches. Paleolimnology: Overview of Paleolimnology; PigmentStudies.

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