9
A new paleoclimate classication for deep time Laiming Zhang a,b , Chengshan Wang a,b, , Xianghui Li c , Ke Cao d , Ying Song e , Bin Hu a,b , Dawei Lu f , Qian Wang a,b , Xiaojing Du a,b , Shuo Cao a,b a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China b School of the Earth Science and Resources, China University of Geosciences, Beijing 100083, China c State Key Laboratory of Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China d The Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Qingdao Institute of Marine Geology, Qingdao 266071, China e School of Geosciences, China University of Petroleum, Qingdao 266580, China f College of Geological Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China abstract article info Article history: Received 26 September 2015 Received in revised form 23 November 2015 Accepted 24 November 2015 Available online 7 December 2015 In deep time, climates are mainly classied by climatically sensitive deposits, paleontological evidences, and modeling. However, they only have limited applicability in deep time studies. Here, we propose a new paleoclimate classication based on the widely used Köppen climate classication. The proposed new classica- tion is simple and quantitative, but bridges the gap between modern and deep time climate studies. The new classication is closely related to but differs from that of Köppen by changing some limits. A world map using the new classication shows the same patterns as the world map of the Köppen climate classication. Using the new classication, we are able to solve a long-standing problem about the climates of East Asia during the Eocene. We found that East Asia shared the same climate type (Ca: Subtropical) at all studied locations, supporting the hypothesis of monsoon or monsoon-like climate that prevailed there during the Eocene. © 2015 Elsevier B.V. All rights reserved. Keywords: Paleoclimate classication Deep time Köppen climate classication 1. Introduction Systematic grouping of climates into different types based on partic- ular attributes brings structure, order and simplicity to a complex cli- matic system, which allows us to set spatial boundaries to conditions on Earth's surface (Oliver, 2005). A variety of classications have been established for modern climates based on specic applications (Essenwanger, 2001; Farmer and Cook, 2013). However, paleoclimatol- ogists have found it difcult to apply these modern climate classica- tions to deep time (pre-Quaternary), and there are no widely accepted paleoclimate classications. Deep time climates present special problems for classication, be- cause the instrumental meteorological parameters are totally lacking, such as the temperature, precipitation, wind, and air pressure. All we know about the paleoclimate comes from indirect evidence from the geologic records, i.e., the proxies. However, interpretations of indirect evidence are limited because of our incomplete knowledge on the mea- surements of proxies and relatively poor understanding of climate dy- namics in the past. In consequence, paleoclimate information often has no direct relation to climatic variables used for modern climate clas- sication, a gap between modern and deep time climate studies. Clearly, a paleoclimate classication should be simple enough to de- scribe the limited climatic data available in deep time studies and be re- lated to modern climates, thus serving as a bridge over the gap between modern and deep time climate. More importantly, the paleoclimate classication should be quantitative, with the same measures of modern climates when studying geological ages and regions. We need to clarify that there is no natural boundary in the world that is able to distinctive- ly dene two climate types. Although the boundary between types is quantitatively dened, what the climate classication denes is in fact to show the overall characteristics. Here we propose a new classication for paleoclimates in deep time based on these considerations. The new classication is established based on the widely used Köppen climate classication. However, what we need to note is that the extinct climatein deep time cannot be discriminated in the new climate classication. 2. Terminology In this paper, deep timerefers to the pre-Quaternary, the part of Earth's history that has to be reconstructed from rock, and is older than historical or ice core records (Soreghan, 2005; Montañez et al., 2011). In a narrow sense, climate can be considered as the average weath- erfor 30 years. In a wider sense, climate is the state of all the statistical description of the climate system (Farmer and Cook, 2013). Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98106 Corresponding author at: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China. E-mail address: [email protected] (C. Wang). http://dx.doi.org/10.1016/j.palaeo.2015.11.041 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Palaeogeography, Palaeoclimatology, Palaeoecology · main type and the second and third letters indicating the subtypes. 4.4.1. The first letter The criteria of the A, C, D and E

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Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo

A new paleoclimate classification for deep time

Laiming Zhang a,b, Chengshan Wang a,b,⁎, Xianghui Li c, Ke Cao d, Ying Song e, Bin Hu a,b, Dawei Lu f,Qian Wang a,b, Xiaojing Du a,b, Shuo Cao a,b

a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, Chinab School of the Earth Science and Resources, China University of Geosciences, Beijing 100083, Chinac State Key Laboratory of Mineral Deposit Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinad The Key Laboratory of Marine Hydrocarbon Resources and Environment Geology, Qingdao Institute of Marine Geology, Qingdao 266071, Chinae School of Geosciences, China University of Petroleum, Qingdao 266580, Chinaf College of Geological Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China

⁎ Corresponding author at: State Key Laboratory of BGeology, China University of Geosciences, Beijing 100083

E-mail address: [email protected] (C. Wang).

http://dx.doi.org/10.1016/j.palaeo.2015.11.0410031-0182/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 September 2015Received in revised form 23 November 2015Accepted 24 November 2015Available online 7 December 2015

In deep time, climates are mainly classified by climatically sensitive deposits, paleontological evidences, andmodeling. However, they only have limited applicability in deep time studies. Here, we propose a newpaleoclimate classification based on the widely used Köppen climate classification. The proposed new classifica-tion is simple and quantitative, but bridges the gap between modern and deep time climate studies. The newclassification is closely related to but differs from that of Köppen by changing some limits. A world map usingthe new classification shows the same patterns as the world map of the Köppen climate classification. Usingthe new classification, we are able to solve a long-standing problem about the climates of East Asia during theEocene. We found that East Asia shared the same climate type (Ca: Subtropical) at all studied locations,supporting the hypothesis of monsoon or monsoon-like climate that prevailed there during the Eocene.

© 2015 Elsevier B.V. All rights reserved.

Keywords:Paleoclimate classificationDeep timeKöppen climate classification

1. Introduction

Systematic grouping of climates into different types based on partic-ular attributes brings structure, order and simplicity to a complex cli-matic system, which allows us to set spatial boundaries to conditionson Earth's surface (Oliver, 2005). A variety of classifications have beenestablished for modern climates based on specific applications(Essenwanger, 2001; Farmer and Cook, 2013). However, paleoclimatol-ogists have found it difficult to apply these modern climate classifica-tions to deep time (pre-Quaternary), and there are no widely acceptedpaleoclimate classifications.

Deep time climates present special problems for classification, be-cause the instrumental meteorological parameters are totally lacking,such as the temperature, precipitation, wind, and air pressure. All weknow about the paleoclimate comes from indirect evidence from thegeologic records, i.e., the proxies. However, interpretations of indirectevidence are limited because of our incomplete knowledge on themea-surements of proxies and relatively poor understanding of climate dy-namics in the past. In consequence, paleoclimate information oftenhas no direct relation to climatic variables used formodern climate clas-sification, a gap between modern and deep time climate studies.

iogeology and Environmental, China.

Clearly, a paleoclimate classification should be simple enough to de-scribe the limited climatic data available in deep time studies and be re-lated tomodern climates, thus serving as a bridge over the gap betweenmodern and deep time climate. More importantly, the paleoclimateclassification should be quantitative,with the samemeasures ofmodernclimates when studying geological ages and regions. We need to clarifythat there is no natural boundary in theworld that is able to distinctive-ly define two climate types. Although the boundary between types isquantitatively defined, what the climate classification defines is in factto show the overall characteristics.

Here we propose a new classification for paleoclimates in deep timebased on these considerations. The new classification is establishedbased on the widely used Köppen climate classification. However,what we need to note is that the ‘extinct climate’ in deep time cannotbe discriminated in the new climate classification.

2. Terminology

In this paper, ‘deep time’ refers to the pre-Quaternary, the part ofEarth's history that has to be reconstructed from rock, and is olderthan historical or ice core records (Soreghan, 2005; Montañez et al.,2011).

In a narrow sense, climate can be considered as the “average weath-er” for 30 years. In a wider sense, climate is the state of all the statisticaldescription of the climate system (Farmer and Cook, 2013).

99L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

Climate classification is a systematic arrangement, gathering cli-mates into groups or categories using boundaries defined by similarconditions and meteorological elements. In this study, the discussionof classifications is limited to global and regional climates.

3. Previous paleoclimate classifications

In many previous studies, paleoclimates have been classified byclimatically sensitive deposits, paleontologic evidence, and by numericalmodeling.

Since the middle of the last century, many researchers haveattempted to reconstruct climate zones in deep time by analyzing thedistribution of climatically sensitive deposits (Strakhov, 1967;Bárdossy, 1982; Hallam, 1984, 1985). Deposits indicative of special con-ditions, such as evaporites, calcretes, tillites, laterites and bauxites, arewidely distributed both in time and space. They have been used to clas-sify paleoclimates in the early Phanerozoic before the rise of land plantsor animals (Boucot et al., 2013), and younger times (Chumakov, 2004;Winguth and Maier-Reimer, 2005; Guo et al., 2008; Dera et al., 2009;Boucot et al., 2013). Most of these indicators are qualitative, althoughsome of them can be interpreted as semi-quantitative (Tabor andPoulsen, 2008; Craggs et al., 2012). However, climate classificationsbased on these commonly have nomore than 5 climatic zones on a glob-al scale, and even fewer on regional scales. Such low-resolution classifi-cations cannot always provide enough climatic information forpaleoclimate studies.

Palynological andmacro-plantmaterials have been extensively usedto evaluate ancient climatic conditions (Parrish, 1998; Royer, 2012;Boucot et al., 2013). The abundance, diversity and distribution of vege-tation types (Larsson et al., 2010), morphology and structure of theplant, especially the leaf physiognomy (Wolfe, 1995; Wilf, 1997; Wilfet al., 1998; Spicer, 2012) are valuable climate indicators. It is generallyassumed that the conditions under which a plant lived were similar tothose of its nearest living relatives (NLRs) (Vakhrameev et al., 1991;Iannuzzi and Rösler, 2000; Sun and Wang, 2005; Fernández et al.,2007; Iglesias et al., 2011).

Many studies have used the distribution of invertebrates to definebiogeographic provinces. Among the aquatic invertebrate fossils, themorphology, abundance, diversity and distribution of ostracods (Denget al., 2010, 2012), conchostracans (personal communication withGang Li), and bivalves (Deng et al., 2010, 2012) have been used aspaleoclimate indicators. As with plant fossils, their paleoclimatic inter-pretation is based on the tolerances of their modern counterparts. Inpractice, information from invertebrate fossils is always combinedwith other paleoclimatic indicators, such as fossil plants and climaticallysensitive deposits (Boucot et al., 2013).

An important concept in climate classification is that “the vegetationis the best expression of climate” (Kottek et al., 2006; Peel et al., 2007;Köppen, 1884). Each paleovegetation type represents a set of paleocli-matic conditions (Haxeltine and Prentice, 1996; DeConto et al., 2000;Foley et al., 2000; Bergengren et al., 2001; Walter, 2002; Kaplan et al.,2003). Paleoclimate changes thus can be interpreted from the changesof the paleovegetation, and their interpretations are generally consis-tent with other evidence. However, the ages of the paleofloras areoften uncertain, and the number of recognizable vegetation types is lim-ited and the topographic resolution of the environment in which theylived is largely unclear.

Numerical paleoclimate models start with a specific set of boundaryconditions and then calculate atmospheric and oceanic conditions atspecific time intervals. The model outputs are usually in the averageconditions of temperature, precipitation, and evaporation. Themodeledpaleoclimates can be compared directlywith theirmodern counterparts(Otto-Bliesner and Upchurch, 1997; Upchurch et al., 1998; Roscheret al., 2011; Tang et al., 2011; Zhang et al., 2012; De Vleeschouweret al., 2014; Gulbranson et al., 2014). The models commonly includepaleovegetation simulations of varying complexity. However, the

results of numerical modeling may contradict the geological proxies,as is the case with the ‘cold continental interior’ paradox for the LateCretaceous (DeConto et al., 1999). Such model-data discrepancies maybe due to incorrect assumption in the initial boundary conditions,poor model resolution, or incomplete representation of the relevantphysics (Huber, 2012).

4. A new paleoclimate classification for deep time

4.1. Method

Wemodify Köppen climate classification to adapt it to deep time. Toaccomplish this, we used the following steps: 1) determining thepaleoclimate parameters that are available in deep time studies; 2) in-vestigating the relation of these paleoclimate parameters to themodernclimate types recognized in Köppen climate classification; 3) redefiningthe climate types and their boundaries by these paleoclimate parame-ters; and 4) testing and verifying the new paleoclimate classification.

In the new climate classification, the threshold values for the bound-aries are determined based on the principle that the threshold value canminimize the misallocation of observed Köppen climate at each stationinto the new groups.

4.2. Köppen climate classification

In the late 19th century Köppen proposed the first quantitative clas-sification ofworld climates (Kottek et al., 2006), and it remains themostwidely used (Rubel and Kottek, 2011). It is based on the idea that thevegetation is the best expression of long-term climate conditions. Theboundaries are a hierarchical system related to vegetation distributionsthat reflect major climate variables. Köppen climate classification com-bines average annual/monthly temperatures, average annual/monthlyprecipitation, and seasonality of precipitation (Kottek et al., 2006; Peelet al., 2007).

The classification is a hierarchy that starts by recognizing five majorclimates, denoted by letters: A = Tropical; B = Arid; C = Temperate;D = Continental; and E = Polar (Table 1). A, C, D and E are defined bytemperature only; B is defined by the combination ofminimal precipita-tion and temperature. In practice, the E climate is determined first,followed by B and then the A, C, and D climates.

Eachmajor climate type is then subdivided using precipitation, indi-cated by a second letter: W=Desert; S = Steppe; f = fully humid; s =summer dry; w=winter dry; m=monsoonal. These units can then befurther subdivided using temperature, indicated by a third letter: h =hot arid; k= cold arid; F= polar frost; T= polar tundra; a= hot sum-mer; b=warm summer; c= cool summer; d= extremely continental(Table 1).

Each climate type is thus represented by a 2-to-3-letter symbol sothat climatologists can choose an appropriate level of complexitybased on their scientific objectives and the nature of the climate dataavailable.

The description and criteria of Köppen climate types are in Table 1,following Peel et al. (2007) update. According to this, thirty climatetypes in total are recognized in modern climate: 3 Tropical (Af, Am,and As/Aw), 4 Arid (BWh, BWk, BSh, and BSk), 9 Temperate (Csa, Csb,Csc, Cfa, Cfb, Cfc, Cwa, Cwb, and Cwc), 12 Continental (Dsa, Dsb, Dsc,Dsd, Dfa, Dfb, Dfc, Dfd, Dwa, Dwb, Dwc, and Dwd) and 2 Polar (ET andEF).

4.3. Parameters and data

Köppen climate types are defined by a complex combination of tem-perature, precipitation, and seasonality information, such as meanmonthly temperature, maximum andminimummonthly temperatures,lowest and highest monthly precipitation values for the summer andwinter half-years, and dryness threshold (Table 1). However, most of

Table 1The description and the criteria of Köppen's climate classification.a

Modified after Peel et al. (2007).

Symbol Major climate Criteria

A Tropical Tmin ≥ 18Af Tropical rainforest Pmin ≥ 60Am Tropical monsoon Not (Af) & Pmin ≥ 100-MAP/25Aw/As Tropical savannah Not (Af) & Pmin b 100-MAP/25

B Dry MAP b 10 × PthBS Steppe 5 ≤ MAP b 10 × PthBSh Hot steppe MAT ≥ 18BSk Cold steppe MAT b 18

BW Desert MAP b 5 × PthBWh Hot desert MAT ≥ 18BWk Cold desert MAT b 18

C Temperate Tmax N 10 & 0 b Tmin b 18Cs Mediterranean Psmin b 40 & Psmin b Pwmax/3Csa Mediterranean (hot summer) Tmax ≥ 22Csb Mediterranean (warm summer) Not (a) & Tmon10 ≥ 4Csc Mediterranean (cold summer) Not (a or b) & 1 ≤ Tmon10 b 4

Cw Temperate with dry winter Pwmin b Psmax/10Cwa Humid subtropical Tmax ≥ 22Cwb Maritime temperate Not (a) & Tmon10 ≥ 4Cwc Maritime subarctic Not (a or b) & 1 ≤ Tmon10 b 4

Cf Temperate with fully humid Not (Cs or Cw)Cfa Humid subtropical Tmax ≥ 22Cfb Maritime temperate Not (a) & Tmon10 ≥ 4Cfc Maritime subarctic Not (a or b) & 1 ≤ Tmon10 b 4

D Continental Tmax N 10 & Tmin ≤ 0Ds Continental with dry summer Psmin b 40 & Psmin b Pwmax/3Dsa Continental (hot summer) Tmax ≥ 22Dsb Continental (warm summer) Not (a) & Tmon10 ≥ 4Dsc Continental subarctic (cold summer) Not (a or b or d)Dsd Continental subarctic (very cold winter) Not (a or b) & Tmin b −38

Dw Continental with dry winter Pwmin b Psmax/10Dwa Continental (hot summer) Tmax ≥ 22Dwb Continental (warm summer) Not (a) & Tmon10 ≥ 4Dwc Continental subarctic (cold summer) Not (a or b or d)Dwd Continental subarctic (very cold winter) Not (a or b) & Tmin b −38

Df Continental with fully humid Not (Cs or Cw)Dfa Continental (hot summer) Tmax ≥ 22Dfb Continental (warm summer) Not (a) & Tmon10 ≥ 4Dfc Continental subarctic (cold summer) Not (a or b or d)Dfd Continental subarctic (very cold winter) Not (a or b) & Tmin b −38

E Polar Tmax b 10ET Tundra 0 b Tmax b 10EF Ice cap Tmax ≤ 0

a MAT = mean annual temperature, MAP = mean annual precipitation, Tmax =maximum monthly temperature, Tmin = minimum monthly temperature, Tmon10 =number of months where the temperature is above 10, Pmin = minimum monthlyprecipitation, Psmin = minimum monthly precipitation in summer, Pwmin = minimummonthly precipitation in winter, Psmax = maximum monthly precipitation in summer,Pwmax = maximum monthly precipitation in winter. If 70% of MAP occurs in winterthen Pth= 2×MAT, if 70% ofMAP occurs in summer then Pth= 2×MAT+28, otherwisePth = 2 × MAT + 14. Summer (winter) is defined as the warmer (cooler) six monthperiod of ONDJFM and AMJJAS.

Table 2Quantitative methods with acceptable errors in deep time.

Method MATa WMMT MAP AIKöppen Reference

Oxygen isotope (δ18O) √ Grossman (2012)Clumped isotope √ √ Passey et al. (2010)TEX86 √ Schouten et al. (2013)MBT/CBT √ Schouten et al. (2013)Depth of Bk to thepaleosol surface

√ Retallack (2005)

Element geochemistryof Paleosol

√ √ √ Sheldon andTabor (2009)

Coexistenceapproach (CA)

√ √ √ √ Utescher et al. (2007)

Pollen/leaf composition √ √ √ √ Wilf et al. (1998)CLAMPb √ √ √ √ Spicer (2012)

a MAT=mean annual temperature,MAP=mean annual precipitation,WMMT=warmmonths mean temperature, AIKöppen = Köppen aridity index.

b CLAMP = Climate Leaf Analysis Multivariate Program.

100 L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

these climatic parameters are unknown in deep time. Thereforewe herepropose to use four simple parameters: mean annual temperature(MAT), mean annual precipitation (MAP), warm month mean temper-ature (WMMT: defined by warmest monthly mean temperature ofthree consecutive months), and Köppen aridity index (AIKöppen). Indeep time these can be estimated quantitatively using a variety ofmethods with acceptable errors (Table 2).

The Köppen aridity index (AIKöppen) is not often cited, but it has thehighest accuracy and precision among the many aridity indices (Quanet al., 2013). It is calculated by MAP/(MAT + 33) (Köppen, 1923).When AIKöppen b 5.7, the climate is considered to be arid, and when5.7 ≤ AIKöppen b 13.6, the climate is considered to be semi-arid (Quanet al., 2013).

The global climatic data used in the present study are from theGlob-al Historical ClimatologyNetwork (GHCN) version 2.0 dataset (Petersonand Vose, 1997). They are based on stations at 4279 locations recorded

each month (Table S1). The new classification of modern climates isbased on the work of Peel et al. (2007).

4.4. New paleoclimate classification

The paleoclimate classification we propose here utilizes the hierar-chy of Köppen climate classification, with the first letter indicating themain type and the second and third letters indicating the subtypes.

4.4.1. The first letterThe criteria of the A, C, D and E climates are mutually exclusive and

are defined based on the maximum and minimum monthly tempera-tures (Tmax and Tmin) (Table 1). The MAT can be used as a substitutefor Tmax and Tmin (Fig. 1). Generally, theMAT of the A climate is highest,then C and D with the MAT of E being the lowest. In reality all theboundaries are vague and gradual (Fig. 1a) and the boundaries aredrawn somewhat arbitrarily. MATs of 23 °C, 9 °C, and−10 °C are usedto define the boundaries between the A and C climates, the C and D cli-mates, and the D and E climates, respectively (Fig. 1b). The correlationbetween the MAT and the minimum monthly temperature (Tmin) ispositively strong (R2 = 0.9280, p b 0.01; Fig. 1c).

In the new classification, areas with a MAT no lower than 23 °C areassigned to the A climate; with a MAT between 9 °C and 23 °C to the Cclimate; with a MAT between −10 °C and 9 °C to the D climate, andthose with a MAT lower than −10 °C to the category E (Table 3).

In the Köppen climate classification, the B climate is defined by thedryness threshold (Pthreshold/Pth). This is calculated by one of the threefunctions of mean annual temperature and mean annual precipitation;the forms of the function depend on the annual distribution of precipi-tation (Table 1). TheMAT,MAP, and the AIKöppen have been investigatedto determine if any of themmight be substituted for the more complexcriteria. These three parameters are plotted against the latitudes of theclimatic data in Figs. 2 and 3.

TheMAT alone is not enough to define the B climate, because there isno distinct difference between the B climate data and other climate datain MAT (Fig. 2a), and the correlation between the MAT and the oldcriteria is weak (Fig. 3a).

TheMAP and AIKöppen show similar distributions. In Fig. 2b and c, thedata are divided into two distinct groups, and the non-B climate data aregenerally higher than the B climate data in both hemispheres. The cor-relation analysis shows similar results; the new criteria (MAP orAIKöppen) and the old criteria are strongly correlated with each other(R2 = 0.9778, p b 0.01 for MAP and R2 = 0.9566, p b 0.01 for AIKöppen)(Fig. 3b and c). However, the data points tend to be divergent when to-wards small values (theB climates) in theMAPfigure,whereas this phe-nomenon is not seen in the AIKöppen data (Fig. 3b and c). Therefore, theAIKöppen seems to be themost suitable parameter to define the B climate.

Fig. 1. Latitudinal distributions (positive/negative values indicate north/south latitudes) of the mean annual temperatures (MAT) according to a) the Köppen climate classification andb) the new paleoclimate classification. The A, C, D, and E climates are represented by red, green, yellow, and blue circles. The MAT values of 23 °C, 9 °C and −10 °C (dotted lines) arethe boundaries between adjacent climate types. c) Correlation between the mean annual temperatures (MAT) and the minimum monthly temperature (Tmin). The former comes fromthe new paleoclimate classification and the latter comes from Köppen's climate classification.

101L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

For the original B climate data, all of the AIKöppen values are smallerthan 13.6. The value of 13.6 is defined as the boundary between semi-arid and humid (Quan et al., 2013). However, if AIKöppen = 13.6 is de-fined as the boundary between the B and non-B climates, many non-Bclimate data will be falsely assigned to the B climate. Therefore, thisvalue needs to be revised to make the distributions of data consistentwith the original distribution to the maximum extent. We comparedthe data distribution based on AIKöppen value between 5.7 and 13.6,and the AIKöppen value of 10.4 is defined as the boundary between theB climate and non-B climates, because only a few non-B data areassigned to the B climate using the value of 10.4, and vice versa. AIKöppenvalues of 5.7 can be used as the boundary between the BS and BW cli-mates. Again, a small amount of BW climate data will be incorrectlyassigned to the BS climate, and vice versa.

Sites with AIKöppen values between 5.7 and 10.4 will be assigned tothe BS climate, and those with AIKöppen values smaller than 5.7 will beassigned to the BW climate (Table 3).

4.4.2. The second letterThe second letters (W, S, f, m, s, andw) describe the amount and the

distribution of the precipitation. W and S define the subtypes of the Bclimate already discussed in Section 4.4.1.

Table 3The description and the criteria of the new paleoclimate classification.

Symbol Major climate Criteriaa

A Tropical MAT ≥ 23Af/Am Tropical Rainforest MAP ≥ 1800As/Aw Tropical savannah MAP b 1800

B Dry AIköppen b 10.4BS Steppe 5.7 ≤ AIköppen b 10.4BSh Hot steppe MAT ≥ 18BSk Cold steppe MAT b 18

BW Desert AIköppen b 5.7BWh Hot desert MAT ≥ 18BWk Cold desert MAT b 18

C Temperate 9 ≤ MAT b 23Ca Humid subtropical WMMT ≥ 21Cb Maritime temperate 15 ≤ WMMT b 21Cc Maritime subarctic WMMT b 15

D Continental −10 ≤ MAT b 9Da Continental (hot summer) WMMT ≥ 21Db Continental (warm summer) 15 ≤ WMMT b 21Dc/Dd Continental subarctic WMMT b 15

E Polar MAT b −10

a MAT=mean annual temperature,MAP=mean annual precipitation,WMMT=warmmonth mean temperature, AIKöppen = Köppen aridity index.

Themean annual precipitation (MAP) is one of few reliablemeasureof precipitation we can obtain in deep time.We can simplify the criteriafor the second letters of the MAP: only one boundary within the Aclimate can be defined. And no subtypes in the C or D climates can berecognized because MAP does not provide any information about theseasonality of precipitation. This single boundary, MAP = 1800 mm, isbetween the Af/Am and the As/Aw climates (Fig. 4; Table 3). Thisvalue can retain the distribution pattern of the Köppen climate classifi-cation in the new classification to the utmost extent.

4.4.3. The third letterThe third letters (T, F, h, k, a, b, c, and d) are defined by themean an-

nual andmonthly temperature. T and F define the subtypes of the E cli-mate (ET and EF). These are not included in the new classificationbecause they are indistinguishable using the available paleoclimatic pa-rameters. The letters h and k designate subtypes of the B climate (BWh,BWk, BSh, and BSk), they are defined by MAT in Köppen climate classi-fication. Therefore, the new classification follows the original definitionswithout modifications (Table 3). The rest of the letters (a, b, c, andd) define subtypes of the C and D climates. Although introduced asthird letter, they can also be used as second letters. For example, theDfc, Dwc, and Dsc climates taken together are defined as a Dc climate(Continental subarctic climates).

In Köppen climate classification, the letters a, b, c, and d are definedby the maximum monthly temperature (Tmax) and the number ofmonths where the temperature is above 10 °C. Therefore, we try to em-ploy the warm months mean temperature (WMMT) to differentiatethem. TheWMMT can be replaced by the summer average temperature(SAT) or maximum monthly temperature (Tmax) if the WMMT is notavailable. The correlation between the WMMT and Tmax is excellent(R2 = 0.9904, p b 0.01; Fig. 5c). The data generally show a decreasefroman ‘a’ climate to a ‘d’ climate both in the C andD climates. However,the boundaries between these climate subtypes are vague and gradual(Fig. 5a). We draw the boundaries arbitrarily, with as small alterationsto the original distributions as possible. Accordingly, WMMTs of 21 °Cand 15 °C are defined as the boundaries of the ‘a’ and ‘b’ climates andthe ‘b’ and ‘c/d’ climates (the c and d climates are combined as c/d inthe new classification) in both the C and D climates, respectively(Fig. 5b).

For the B climate, locations having a MAT no lower than 18 °C areassigned to the Bh climate, and locations having a MAT lower than18 °C, are assigned to the Bk climate. In the C and D climates, locationshaving a WMMT no lower than 21 °C are assigned to the Ca or Da cli-mates. However if the maximum monthly temperature (Tmax) is inuse, the boundary between a and b is 22 °C instead of 21 °C. Locations

Fig. 2. Latitudinal distributions (positive/negative values indicate north/south latitudes) of a) the mean annual temperatures (MAT), b) the mean annual precipitation (MAP), and c) theKöppen arid index (AIKöppen). The B climates and non-B climates are represented by green and red circles. Two data points beyond the scales are indicated by the arrows and values.

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having a WMMT between 15 °C and 21 °C are assigned to the Cb or Dbclimates, and locations with a WMMT lower than 15 °C are assigned tothe Cc or Dc/Dd climates (Table 3).

4.4.4. New paleoclimate classificationIn the new paleoclimate classification the number of climate types is

reduced to 13: 2 Tropical (Af/Am and As/Aw), 4 Arid (BWh, BWk,BSh, and BSk), 3 Temperate (Ca, Cb, and Cc), 3 Continental (Da, Db,and Dc/Dd) and 1 Polar (E). These are sufficient for deep timepaleoclimate studies.

A world map using the new classification has been made followingthe suggestions of Peel et al. (2007) (Fig. 6). Stations from (GHCN) ver-sion 2.0 dataset (Peterson and Vose, 1997) with at least 30 observationsfor each month were used in the analysis; this involves 12,396 precipi-tation stations and 4,844 temperature stations. Where available, themean annual temperature (MAT), mean annual precipitation (MAP),warm month mean temperature (WMMT), and Köppen aridity index(AIKöppen) were calculated for each station. Then a two-dimensional(latitude and longitude) thin-plate spline interpolation with tensionwas applied to each parameter onto a 0.1 × 0.1° grid. All the interpola-tions are performed in ESRI ArcMap version 10.2 using settings of“weight” = 1 and “points” = 10. The new criteria were then appliedto the splined parameters. As shown in Fig. 6, the global configurationof the new world map closely matches the overall pattern of Köppensystem. Note that in the world map of the Köppen climate classificationthe climate types are combined and reduced to 13.

In the world map of the new climate classification, the areas of theAf/Am climate (Tropical rainforest climate and Tropical monsoon cli-mate) commonly straddle the equator, between 5° N and 5° S. The rep-resentative regions are Southeast Asia, Central and West Africa, and

Fig. 3.Correlation betweenMAP-5Pth from theKöppen climate classification and a)mean annulSee Table 1 for detailed explanations of these indicators.

South and Central America. The areas of the As/Aw climate (Tropical sa-vanna climate) are usually located in the outer margins of the Tropicalclimates. The representative regions are located in Africa, SoutheastAsia and South America. Rainforests are recognized in both Köppenand new classification: the North Pacific temperate rainforest, the Ama-zon tropical rainforest, and the Congo River Basin tropical rainforest.

Fig. 6 shows the areas of the BW (Desert) climate to be located be-tween 30° N and 30° S. The deserts are easily recognized along thezonal areas including North Africa, Arabian Peninsula, and Middle East.In addition, they occupy nearly all the Central–West Australia andsome small patches of South Africa and North America. The BS climate(Steppe climate) is commonly located at the outer margins of the BWclimate areas; most are located in Asia and North America. Comparedto the Köppen climate classification map, the areas of BS climate are ex-panded, especially in Asia. The BS climate is the intermediate betweenthe arid (BW climate) and humid climates and is a combination of thearea of these two climate types. The Köppen aridity index (AIKöppen) isaffected by the mixed climates, although we revised the AIKöppen valuefrom 13.6 to 10.4.

The areas of the Ca climate (Humid subtropical climate) are mainlyin the southeast of North America, the south of South America, EastAsia, and Southern Europe. The areas of the Cb/Cc (Maritime temper-ate/subarctic climate) are mostly in Northern Western Europe, withsome isolated areas in southwestern South America and South Africa.The Mediterranean climate is undistinguishable in the new worldmap, because of the distribution of the precipitation cannot be wellconstrained in deep time. The areas shown as Mediterranean climatein Köppen climate classification, such as the area around theMediterra-nean Sea, much of California, and parts of Western and South Australia,are assigned to other C climates in the new climate classification.

temperatures, b)mean annual precipitation (MAP), and c) Köppen aridity index (AIKöppen).

Fig. 4. Latitudinal distributions (positive/negative values indicate north/south latitudes) ofthe mean annual precipitation (MAP) according to the Köppen climate classification. TheAf/Am and As/Aw climates are represented by red and green circles. The MAP value of1800 mm (dotted line) is the boundary between the Af/Am and As/Aw climate types ac-cording to the new paleoclimate classification. Only A climates are plotted in this figure.

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Compared to the Köppen world map, the areas of the Ca climate are ex-panded in North America and Africa, and reduced in Asia in the newworld map.

The distribution of the D climate (Continental climate) is parallel tolatitude. The areas of the Da/Db (Continental climates) are above 40° Nto 65° N latitude, within central and northeastern North America,Europe, and Asia. The areas of the Dc/Dd (subarctic climate) are gener-ally at latitudes from 50° to 70°N, poleward of the Continental climates.In the new map, the areas of the Da climate are often replaced by C cli-mates and areas of the Dc/Dd climates are replaced by the Db climate.This is because the new classification tends to emphasize warmth forthese climate types.

The two major areas with E climate (Ice Cap and Tundra climates)are Antarctica and Greenland. In addition, most northern islands ofCanada and high-latitude Russia also belong to this type. We do nottake altitude specifically into account because of the uncertainties ofpaleoaltimetry. Some areas with high altitudes (above the snow line)can be assigned to the E climate, such as the Andes, the Himalaya,Rocky Mountains, and the Alps.

In the new classification, 3575 (83.55%) of 4279 locations retaintheir original major climate types, and 3223 (75.32%) of 4279 locationsretain their original climate types when classified by subtype (Tables 4

Fig. 5. Latitudinal distributions (positive/negative values indicate north/south latitudes) ofwarmb) the new paleoclimate classification. The a, b, c, and d climates are represented by red, greenpaleoclimate classification.WMMTvalues of 21 °C and 15 °C (dotted lines) are the boundaries be(WMMT) and themaximummonthly temperature (Tmax). The former comes from the new paleand D climates are plotted in this figure.

and S1). Because most stations retain their Köppen climate types inthe new paleoclimate classification, and the others are usually neigh-boring types, the proposed new classification for paleoclimate can beconsidered a close analog of that of Köppen.

4.5. Application

In previous studies, it has been assumed that the Paleogenepaleoclimate in East Asia was controlled by the planetary wind systemwith a unique pattern of three latitudinal zones (Guo et al., 2008) (seefigures in Quan et al., 2014). Two humid zones (indicated by the pres-ence of coal and oil shales) in the north and south, and an arid zone (in-dicated by red beds and evaporites) in the middle (Wang et al., 1999;Sun and Wang, 2005; Guo et al., 2008). However, there is evidencethat monsoon or monsoon-like conditions prevailed in East Asia duringthat period (Quan et al., 2011, 2012b; Wang et al., 2013; Quan et al.,2014). This brings into question the planetary windmodel and the lati-tudinal zonation, especially what has been interpreted as an arid zone.Can the Paleogene of the East Asia interior be considered an arid climateusing themodern climate classification scheme (Quan et al., 2014)? Un-fortunately, verification is impeded by the paucity of data, in terms ofboth quantity and variety.

The new paleoclimate classification with simple parameters is moreuseful. The paleoclimate of East Asia during the Eocene had been recon-structed based on the coexistence approach (CA) using 66 plant assem-blages byQuan et al. (2012a,b) (Table S2). The 66 paleofloras come from37 fossil sites throughout China, with ages ranging from early to late Eo-cene. The Köppen aridity index (AIKöppen) has been calculated for eachplant assemblage based on the reconstructed MAT and MAP. All of theKöppen aridity index (AIKöppen) values are larger than the semi-aridthreshold value of 10.4, and there are no obvious discrepancies betweenthe plant assemblages in the humid and arid zones throughout the Eo-cene (Table S2). All of the plant assemblages represent Ca climates,but two of them can also be classified As/Aw and another two classifiedas Cb climate according to their ranges ofMAT orWMMT. This pattern issimilar to the distribution of climate types in modern China, supportingthe idea of a monsoon or monsoon-like climatology (Quan et al., 2011,2012b; Wang et al., 2013; Quan et al., 2014).

5. Summary

Modern climate classifications do not work well in deep time due tothe inherent differences of climates in the past. Climatically sensitivedeposits, paleontological evidence, and modeling have been employedto describe paleoclimates in deep time. However, they have limited

monthmean temperature (WMMT) according to a) the Köppen climate classification and, yellow, and blue circles. The c and d climates (yellow circles) are combined in the newtween adjacent climate types. c) Correlation between thewarmmonthmean temperatureoclimate classification and the latter comes from the Köppen climate classification. Only C

Fig. 6. Comparison between a) the world map of the new paleoclimate classification and b) the world map based on Köppen's climate classification, modified after Peel et al., 2007.

104 L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

applicability, because of our incomplete knowledge of the measure-ments of proxies and relatively poor understanding of climate dynamicsin the past. We propose a new paleoclimate classification based on amodification of the Köppen climate classification. It is a simplificationof the complex modern climate classification using simple quantitativecriteria. The new classification is closely related to but differs fromthat of Köppen by changing some limits. It can serve to bridge the gapbetween the modern and deep time climate studies. A world mapusing the new classification shows the same patterns as the worldmap of the Köppen climate classification.

Using the new classification, we were able to resolve a long-standing dispute about the climate patterns of East Asia during thePaleogene. The new classification indicates that during the entire

Eocene, all of East Asia shared the same climate type (Ca: Subtropi-cal), supporting the monsoon or monsoon-like conditions in EastAsia during that time, rather than a system controlled by the plane-tary wind.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.palaeo.2015.11.041.

Acknowledgments

Wewould thankWilliamW.Hay,who reviewed ourmanuscript andgave us many useful comments. Laiming Zhang is supported by a schol-arship by the Chinese Scholarship Council. This study was financially

Table 4The distribution of the climate types in the new paleoclimate classification.a

A B C D E Total

Koppen's 398 872 1753 1202 54 4279Paleoclimate 443 1017 1719 1066 34 4279Unchangedb 372 787 1489 907 20 3575Percentagec 93.47% 90.25% 84.94% 75.46% 37.04% 83.55%

Af/Am As/Aw BWh BWk BSh BSk Ca Cb Cc Da Db Dc/Dd E Total

Koppen's 166 232 184 104 188 396 1200 536 17 274 679 249 54 4279Paleoclimate 152 291 183 196 145 493 1316 367 36 74 693 299 34 4279Unchanged 130 200 167 104 113 279 1092 332 1 73 551 161 20 3223Percentage 78.31% 86.21% 90.76% 100.00% 60.11% 70.45% 91.00% 61.94% 5.88% 26.64% 81.15% 64.66% 37.04% 75.32%

a Stations from the Global Historical Climatology Network (GHCN) version 2.0 dataset (Peterson and Vose, 1997). Both precipitation and temperature with at least 30 observations foreach month were recorded at a total of 4279 locations (Table S1).

b The number of locations which have the same climate types in Koppen's climate classification and the new paleoclimate classification.c The number of unchanged locations divided by the number of total locations in Koppen's climate classification.

105L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

supported by the National Basic Research Program of China (973 Pro-ject) 2012CB822000.

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