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

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Palaeogeography, Palaeoclimatology, Palaeoecology

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A new paleoclimate classification for deep time

Laiming Zhang a,b, Chengshan Wang a,b,⁎, Xianghui Li c,KeCaod,YingSonge, Bin Hu a,b,DaweiLuf, Qian Wang a,b, Xiaojing Du a,b,ShuoCaoa,b a State Key Laboratory of Biogeology and Environmental Geology, University of Geosciences, 100083, China b School of the 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 article info abstract

Article history: In deep time, are mainly classified by climatically sensitive deposits, paleontological evidences, and Received 26 September 2015 modeling. However, they only have limited applicability in deep time studies. Here, we propose a new Received in revised form 23 November 2015 paleoclimate classification based on the widely used Köppen classification. The proposed new classifica- Accepted 24 November 2015 tion is simple and quantitative, but bridges the gap between modern and deep time climate studies. The new Available online 7 December 2015 classification is closely related to but differs from that of Köppen by changing some limits. A world map using fi fi Keywords: the new classi cation shows the same patterns as the world map of the Köppen climate classi cation. Using fi Paleoclimate classification the new classi cation, we are able to solve a long-standing problem about the climates of East during the Deep time Eocene. We found that East Asia shared the same climate type (Ca: Subtropical) at all studied locations, Köppen climate classification supporting the hypothesis of monsoon or monsoon-like climate that prevailed there during the Eocene. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Clearly, a paleoclimate classification should be simple enough to de- scribe the limited climatic data available in deep time studies and be re- Systematic grouping of climates into different types based on partic- lated to modern climates, thus serving as a bridge over the gap between ular attributes brings structure, order and simplicity to a complex cli- modern and deep time climate. More importantly, the paleoclimate matic system, which allows us to set spatial boundaries to conditions classification should be quantitative, with the same measures of modern on Earth's surface (Oliver, 2005). A variety of classifications have been climates when studying geological ages and regions. We need to clarify established for modern climates based on specific applications that there is no natural boundary in the world that is able to distinctive- (Essenwanger, 2001; Farmer and Cook, 2013). However, paleoclimatol- ly define two climate types. Although the boundary between types is ogists have found it difficult to apply these modern climate classifica- quantitatively defined, what the climate classification defines is in fact tions to deep time (pre-Quaternary), and there are no widely accepted to show the overall characteristics. paleoclimate classifications. Here we propose a new classification for paleoclimates in deep time Deep time climates present special problems for classification, be- based on these considerations. The new classification is established cause the instrumental meteorological parameters are totally lacking, based on the widely used Köppen climate classification. However, such as the temperature, , wind, and air pressure. All we what we need to note is that the ‘extinct climate’ in deep time cannot know about the paleoclimate comes from indirect evidence from the be discriminated in the new climate classification. 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- 2. Terminology namics in the past. In consequence, paleoclimate information often has no direct relation to climatic variables used for modern climate clas- In this paper, ‘deep time’ refers to the pre-Quaternary, the part of sification, a gap between modern and deep time climate studies. 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- ⁎ Corresponding author at: State Key Laboratory of Biogeology and Environmental ” Geology, China University of Geosciences, Beijing 100083, China. er for 30 . In a wider sense, climate is the state of all the statistical E-mail address: [email protected] (C. Wang). description of the climate system (Farmer and Cook, 2013).

http://dx.doi.org/10.1016/j.palaeo.2015.11.041 0031-0182/© 2015 Elsevier B.V. All rights reserved. L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106 99

Climate classification is a systematic arrangement, gathering cli- results of numerical modeling may contradict the geological proxies, mates into groups or categories using boundaries definedbysimilar as is the case with the ‘cold continental interior’ paradox for the Late conditions and meteorological elements. In this study, the discussion Cretaceous (DeConto et al., 1999). Such model-data discrepancies may of classifications is limited to global and regional climates. be due to incorrect assumption in the initial boundary conditions, poor model resolution, or incomplete representation of the relevant 3. Previous paleoclimate classifications physics (Huber, 2012).

In many previous studies, paleoclimates have been classified by 4. A new paleoclimate classification for deep time climatically sensitive deposits, paleontologic evidence, and by numerical modeling. 4.1. Method Since the middle of the last century, many researchers have attempted to reconstruct climate zones in deep time by analyzing the We modify Köppen climate classification to adapt it to deep time. To distribution of climatically sensitive deposits (Strakhov, 1967; accomplish this, we used the following steps: 1) determining the Bárdossy, 1982; Hallam, 1984, 1985). Deposits indicative of special con- paleoclimate parameters that are available in deep time studies; 2) in- ditions, such as evaporites, calcretes, tillites, laterites and bauxites, are vestigating the relation of these paleoclimate parameters to the modern widely distributed both in time and space. They have been used to clas- climate types recognized in Köppen climate classification; 3) redefining sify paleoclimates in the early Phanerozoic before the rise of land the climate types and their boundaries by these paleoclimate parame- or animals (Boucot et al., 2013), and younger times (Chumakov, 2004; ters; and 4) testing and verifying the new paleoclimate classification. Winguth and Maier-Reimer, 2005; Guo et al., 2008; Dera et al., 2009; In the new climate classification, the threshold values for the bound- Boucot et al., 2013). Most of these indicators are qualitative, although aries are determined based on the principle that the threshold value can some of them can be interpreted as semi-quantitative (Tabor and minimize the misallocation of observed Köppen climate at each station Poulsen, 2008; Craggs et al., 2012). However, climate classifications into the new groups. based on these commonly have no more than 5 climatic zones on a glob- al scale, and even fewer on regional scales. Such low-resolution classifi- 4.2. Köppen climate classification cations cannot always provide enough climatic information for paleoclimate studies. In the late 19th century Köppen proposed the first quantitative clas- Palynological and macro- materials have been extensively used sification of world climates (Kottek et al., 2006), and it remains the most to evaluate ancient climatic conditions (Parrish, 1998; Royer, 2012; widely used (Rubel and Kottek, 2011). It is based on the idea that the Boucot et al., 2013). The abundance, diversity and distribution of vege- is the best expression of long-term climate conditions. The tation types (Larsson et al., 2010), morphology and structure of the boundaries are a hierarchical system related to vegetation distributions plant, especially the leaf physiognomy (Wolfe, 1995; Wilf, 1997; Wilf that reflect major climate variables. Köppen climate classification com- et al., 1998; Spicer, 2012) are valuable climate indicators. It is generally bines average annual/monthly temperatures, average annual/monthly assumed that the conditions under which a plant lived were similar to precipitation, and seasonality of precipitation (Kottek et al., 2006; Peel those of its nearest living relatives (NLRs) (Vakhrameev et al., 1991; et al., 2007). Iannuzzi and Rösler, 2000; Sun and Wang, 2005; Fernández et al., The classification is a hierarchy that starts by recognizing five major 2007; Iglesias et al., 2011). climates, denoted by letters: A = Tropical; B = Arid; C = Temperate; Many studies have used the distribution of invertebrates to define D = Continental; and E = Polar (Table 1). A, C, D and E are defined by biogeographic provinces. Among the aquatic invertebrate fossils, the temperature only; B is defined by the combination of minimal precipita- morphology, abundance, diversity and distribution of ostracods (Deng tion and temperature. In practice, the E climate is determined first, et al., 2010, 2012), conchostracans (personal communication with followed by B and then the A, C, and D climates. Gang Li), and bivalves (Deng et al., 2010, 2012) have been used as Each major climate type is then subdivided using precipitation, indi- paleoclimate indicators. As with plant fossils, their paleoclimatic inter- cated by a second letter: W = Desert; S = ; f = fully humid; s = pretation is based on the tolerances of their modern counterparts. In dry; w = dry; m = monsoonal. These units can then be practice, information from invertebrate fossils is always combined further subdivided using temperature, indicated by a third letter: h = with other paleoclimatic indicators, such as fossil plants and climatically hot arid; k = cold arid; F = polar frost; T = polar ; a = hot sum- sensitive deposits (Boucot et al., 2013). mer; b = warm summer; c = cool summer; d = extremely continental An important concept in climate classification is that “the vegetation (Table 1). is the best expression of climate” (Kottek et al., 2006; Peel et al., 2007; Each climate type is thus represented by a 2-to-3-letter symbol so Köppen, 1884). Each paleovegetation type represents a set of paleocli- that climatologists can choose an appropriate level of complexity matic conditions (Haxeltine and Prentice, 1996; DeConto et al., 2000; based on their scientific objectives and the nature of the climate data Foley et al., 2000; Bergengren et al., 2001; Walter, 2002; Kaplan et al., available. 2003). Paleoclimate changes thus can be interpreted from the changes The description and criteria of Köppen climate types are in Table 1, of the paleovegetation, and their interpretations are generally consis- following Peel et al. (2007) update. According to this, thirty climate tent with other evidence. However, the ages of the paleofloras are types in total are recognized in modern climate: 3 Tropical (Af, Am, often uncertain, and the number of recognizable vegetation types is lim- and As/Aw), 4 Arid (BWh, BWk, BSh, and BSk), 9 Temperate (Csa, Csb, ited and the topographic resolution of the environment in which they Csc, Cfa, Cfb, Cfc, Cwa, Cwb, and Cwc), 12 Continental (Dsa, Dsb, Dsc, lived is largely unclear. Dsd, Dfa, Dfb, Dfc, Dfd, Dwa, Dwb, Dwc, and Dwd) and 2 Polar (ET and Numerical paleoclimate models start with a specificsetofboundary EF). conditions and then calculate atmospheric and oceanic conditions at specific time intervals. The model outputs are usually in the average 4.3. Parameters and data conditions of temperature, precipitation, and evaporation. The modeled paleoclimates can be compared directly with their modern counterparts Köppen climate types are defined by a complex combination of tem- (Otto-Bliesner and Upchurch, 1997; Upchurch et al., 1998; Roscher perature, precipitation, and seasonality information, such as mean et al., 2011; Tang et al., 2011; Zhang et al., 2012; De Vleeschouwer monthly temperature, maximum and minimum monthly temperatures, et al., 2014; Gulbranson et al., 2014). The models commonly include lowest and highest monthly precipitation values for the summer and paleovegetation simulations of varying complexity. However, the winter half-years, and dryness threshold (Table 1). However, most of 100 L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

Table 1 Table 2 The description and the criteria of Köppen's climate classification.a Quantitative methods with acceptable errors in deep time. Modified after Peel et al. (2007). a Method MAT WMMT MAP AIKöppen Reference Symbol Major climate Criteria Oxygen isotope (δ18O) √ Grossman (2012)

A Tropical Tmin ≥ 18 Clumped isotope √√ Passey et al. (2010)

Af Tropical rainforest Pmin ≥ 60 TEX86 √ Schouten et al. (2013)

Am Tropical monsoon Not (Af) & Pmin ≥ 100-MAP/25 MBT/CBT √ Schouten et al. (2013)

Aw/As Tropical savannah Not (Af) & Pmin b 100-MAP/25 Depth of Bk to the √ Retallack (2005)

B Dry MAP b 10 × Pth paleosol surface

BS Steppe 5 ≤ MAP b 10 × Pth Element geochemistry √√√Sheldon and BSh Hot steppe MAT ≥ 18 of Paleosol Tabor (2009) BSk Cold steppe MAT b 18 Coexistence √√ √√ Utescher et al. (2007)

BW Desert MAP b 5×Pth approach (CA) BWh Hot desert MAT ≥ 18 Pollen/leaf composition √√ √√ Wilf et al. (1998) BWk Cold desert MAT b 18 CLAMPb √√ √√ Spicer (2012) C Temperate T N 10 & 0 b T b 18 max min a MAT = mean annual temperature, MAP = mean annual precipitation, WMMT = warm Cs Mediterranean P b 40 & P b P /3 smin smin wmax months mean temperature, AI = Köppen aridity index. Csa Mediterranean (hot summer) T ≥ 22 Köppen max b CLAMP = Climate Leaf Analysis Multivariate Program. Csb Mediterranean (warm summer) Not (a) & Tmon10 ≥ 4

Csc Mediterranean (cold summer) Not (a or b) & 1 ≤ Tmon10 b 4 Cw Temperate with dry winter P b P /10 wmin smax fi Cwa Humid subtropical Tmax ≥ 22 each month (Table S1). The new classi cation of modern climates is ≥ Cwb Maritime temperate Not (a) & Tmon10 4 based on the work of Peel et al. (2007). Cwc Maritime Not (a or b) & 1 ≤ Tmon10 b 4 Cf Temperate with fully humid Not (Cs or Cw) ≥ Cfa Humid subtropical Tmax 22 4.4. New paleoclimate classification Cfb Maritime temperate Not (a) & Tmon10 ≥ 4 Cfc Maritime subarctic Not (a or b) & 1 ≤ T b 4 mon10 The paleoclimate classification we propose here utilizes the hierar- D Continental T N 10 & T ≤ 0 max min fi fi Ds Continental with dry summer Psmin b 40 & Psmin b Pwmax/3 chy of Köppen climate classi cation, with the rst letter indicating the

Dsa Continental (hot summer) Tmax ≥ 22 main type and the second and third letters indicating the subtypes.

Dsb Continental (warm summer) Not (a) & Tmon10 ≥ 4 Dsc Continental subarctic (cold summer) Not (a or b or d) b − Dsd Continental subarctic (very cold winter) Not (a or b) & Tmin 38 4.4.1. The first letter b Dw Continental with dry winter Pwmin Psmax/10 The criteria of the A, C, D and E climates are mutually exclusive and ≥ Dwa Continental (hot summer) Tmax 22 are defined based on the maximum and minimum monthly tempera- Dwb Continental (warm summer) Not (a) & Tmon10 ≥ 4 Dwc Continental subarctic (cold summer) Not (a or b or d) tures (Tmax and Tmin)(Table 1). The MAT can be used as a substitute

Dwd Continental subarctic (very cold winter) Not (a or b) & Tmin b −38 for Tmax and Tmin (Fig. 1). Generally, the MAT of the A climate is highest, Df Continental with fully humid Not (Cs or Cw) then C and D with the MAT of E being the lowest. In reality all the ≥ Dfa Continental (hot summer) Tmax 22 boundaries are vague and gradual (Fig. 1a) and the boundaries are ≥ Dfb Continental (warm summer) Not (a) & Tmon10 4 − Dfc Continental subarctic (cold summer) Not (a or b or d) drawn somewhat arbitrarily. MATs of 23 °C, 9 °C, and 10 °C are used fi Dfd Continental subarctic (very cold winter) Not (a or b) & Tmin b −38 to de ne the boundaries between the A and C climates, the C and D cli- E Polar Tmax b 10 mates, and the D and E climates, respectively (Fig. 1b). The correlation ET Tundra 0 b T b 10 max between the MAT and the minimum monthly temperature (Tmin)is ≤ EF Ice cap Tmax 0 positively strong (R2 = 0.9280, p b 0.01; Fig. 1c). a MAT = mean annual temperature, MAP = mean annual precipitation, Tmax = In the new classification, areas with a MAT no lower than 23 °C are maximum monthly temperature, Tmin = minimum monthly temperature, Tmon10 = assigned to the A climate; with a MAT between 9 °C and 23 °C to the C number of months where the temperature is above 10, Pmin = minimum monthly − precipitation, P = minimum monthly precipitation in summer, P = minimum climate; with a MAT between 10 °C and 9 °C to the D climate, and smin wmin − monthly precipitation in winter, Psmax = maximum monthly precipitation in summer, those with a MAT lower than 10 °C to the category E (Table 3). Pwmax = maximum monthly precipitation in winter. If 70% of MAP occurs in winter In the Köppen climate classification, the B climate is defined by the then P = 2 × MAT, if 70% of MAP occurs in summer then P = 2 × MAT + 28, otherwise th th dryness threshold (Pthreshold/Pth). This is calculated by one of the three P = 2 × MAT + 14. Summer (winter) is defined as the warmer (cooler) six month th functions of mean annual temperature and mean annual precipitation; period of ONDJFM and AMJJAS. the forms of the function depend on the annual distribution of precipi-

tation (Table 1). The MAT, MAP, and the AIKöppen have been investigated to determine if any of them might be substituted for the more complex these climatic parameters are unknown in deep time. Therefore we here criteria. These three parameters are plotted against the latitudes of the propose to use four simple parameters: mean annual temperature climatic data in Figs. 2 and 3. (MAT), mean annual precipitation (MAP), warm month mean temper- The MAT alone is not enough to define the B climate, because there is ature (WMMT: defined by warmest monthly mean temperature of no distinct difference between the B climate data and other climate data three consecutive months), and Köppen aridity index (AIKöppen). In in MAT (Fig. 2a), and the correlation between the MAT and the old deep time these can be estimated quantitatively using a variety of criteria is weak (Fig. 3a). methods with acceptable errors (Table 2). The MAP and AIKöppen show similar distributions. In Fig. 2b and c, the The Köppen aridity index (AIKöppen) is not often cited, but it has the data are divided into two distinct groups, and the non-B climate data are highest accuracy and precision among the many aridity indices (Quan generally higher than the B climate data in both hemispheres. The cor- et al., 2013). It is calculated by MAP/(MAT + 33) (Köppen, 1923). relation analysis shows similar results; the new criteria (MAP or b When AIKöppen 5.7, the climate is considered to be arid, and when AIKöppen) and the old criteria are strongly correlated with each other ≤ b 2 2 5.7 AIKöppen 13.6, the climate is considered to be semi-arid (Quan (R = 0.9778, p b 0.01 for MAP and R = 0.9566, p b 0.01 for AIKöppen) et al., 2013). (Fig. 3b and c). However, the data points tend to be divergent when to- The global climatic data used in the present study are from the Glob- wards small values (the B climates) in the MAP figure, whereas this phe- al Historical Climatology Network (GHCN) version 2.0 dataset (Peterson nomenon is not seen in the AIKöppen data (Fig. 3b and c). Therefore, the and Vose, 1997). They are based on stations at 4279 locations recorded AIKöppen seems to be the most suitable parameter to define the B climate. L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106 101

Fig. 1. Latitudinal distributions (positive/negative values indicate north/south latitudes) of the mean annual temperatures (MAT) according to a) the Köppen climate classification and b) 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) are the boundaries between adjacent climate types. c) Correlation between the mean annual temperatures (MAT) and the minimum monthly temperature (Tmin). The former comes from the new paleoclimate classification and the latter comes from Köppen's climate classification.

For the original B climate data, all of the AIKöppen values are smaller The mean annual precipitation (MAP) is one of few reliable measure than 13.6. The value of 13.6 is defined as the boundary between semi- of precipitation we can obtain in deep time. We can simplify the criteria arid and humid (Quan et al., 2013). However, if AIKöppen = 13.6 is de- for the second letters of the MAP: only one boundary within the A fined as the boundary between the B and non-B climates, many non-B climate can be defined. And no subtypes in the C or D climates can be climate data will be falsely assigned to the B climate. Therefore, this recognized because MAP does not provide any information about the value needs to be revised to make the distributions of data consistent seasonality of precipitation. This single boundary, MAP = 1800 mm, is with the original distribution to the maximum extent. We compared between the Af/Am and the As/Aw climates (Fig. 4; Table 3). This the data distribution based on AIKöppen value between 5.7 and 13.6, value can retain the distribution pattern of the Köppen climate classifi- and the AIKöppen value of 10.4 is defined as the boundary between the cation in the new classification to the utmost extent. B climate and non-B climates, because only a few non-B data are assigned to the B climate using the value of 10.4, and vice versa. AIKöppen 4.4.3. The third letter values of 5.7 can be used as the boundary between the BS and BW cli- The third letters (T, F, h, k, a, b, c, and d) are defined by the mean an- mates. Again, a small amount of BW climate data will be incorrectly nual and monthly temperature. T and F define the subtypes of the E cli- assigned to the BS climate, and vice versa. mate (ET and EF). These are not included in the new classification

Sites with AIKöppen values between 5.7 and 10.4 will be assigned to because they are indistinguishable using the available paleoclimatic pa- the BS climate, and those with AIKöppen values smaller than 5.7 will be rameters. The letters h and k designate subtypes of the B climate (BWh, assigned to the BW climate (Table 3). BWk, BSh, and BSk), they are defined by MAT in Köppen climate classi- fication. Therefore, the new classification follows the original definitions 4.4.2. The second letter without modifications (Table 3). The rest of the letters (a, b, c, and The second letters (W, S, f, m, s, and w) describe the amount and the d) define subtypes of the C and D climates. Although introduced as distribution of the precipitation. W and S define the subtypes of the B third letter, they can also be used as second letters. For example, the climate already discussed in Section 4.4.1. Dfc, 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 defined Table 3 by the maximum monthly temperature (Tmax) and the number of The description and the criteria of the new paleoclimate classification. months where the temperature is above 10 °C. Therefore, we try to em- Symbol Major climate Criteriaa ploy the warm months mean temperature (WMMT) to differentiate A Tropical MAT ≥ 23 them. The WMMT can be replaced by the summer average temperature Af/Am Tropical Rainforest MAP ≥ 1800 (SAT) or maximum monthly temperature (Tmax) if the WMMT is not As/Aw Tropical savannah MAP b 1800 available. The correlation between the WMMT and Tmax is excellent b B Dry AIköppen 10.4 (R2 = 0.9904, p b 0.01; Fig. 5c). The data generally show a decrease ≤ b BS Steppe 5.7 AIköppen 10.4 ‘ ’ ‘ ’ BSh Hot steppe MAT ≥ 18 from an a climate to a d climate both in the C and D climates. However, BSk Cold steppe MAT b 18 the boundaries between these climate subtypes are vague and gradual b BW Desert AIköppen 5.7 (Fig. 5a). We draw the boundaries arbitrarily, with as small alterations BWh Hot desert MAT ≥ 18 to the original distributions as possible. Accordingly, WMMTs of 21 °C BWk Cold desert MAT b 18 and 15 °C are defined as the boundaries of the ‘a’ and ‘b’ climates and C Temperate 9 ≤ MAT b 23 ‘ ’ ‘ ’ Ca Humid subtropical WMMT ≥ 21 the b and c/d climates (the c and d climates are combined as c/d in Cb Maritime temperate 15 ≤ WMMT b 21 the new classification) in both the C and D climates, respectively Cc Maritime subarctic WMMT b 15 (Fig. 5b). − ≤ b D Continental 10 MAT 9 For the B climate, locations having a MAT no lower than 18 °C are Da Continental (hot summer) WMMT ≥ 21 Db Continental (warm summer) 15 ≤ WMMT b 21 assigned to the Bh climate, and locations having a MAT lower than Dc/Dd Continental subarctic WMMT b 15 18 °C, are assigned to the Bk climate. In the C and D climates, locations E Polar MAT b −10 having a WMMT no lower than 21 °C are assigned to the Ca or Da cli- a MAT = mean annual temperature, MAP = mean annual precipitation, WMMT = warm mates. However if the maximum monthly temperature (Tmax)isin monthmeantemperature,AIKöppen = Köppen aridity index. use, the boundary between a and b is 22 °C instead of 21 °C. Locations 102 L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

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) the

Köppen 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.

having a WMMT between 15 °C and 21 °C are assigned to the Cb or Db South and Central America. The areas of the As/Aw climate (Tropical sa- climates, and locations with a WMMT lower than 15 °C are assigned to vanna climate) are usually located in the outer margins of the Tropical the Cc or Dc/Dd climates (Table 3). climates. The representative regions are located in , Southeast Asia and South America. Rainforests are recognized in both Köppen 4.4.4. New paleoclimate classification and new classification: the North Pacific temperate rainforest, the Ama- In the new paleoclimate classification the number of climate types is zon tropical rainforest, and the Congo River Basin tropical rainforest. reduced to 13: 2 Tropical (Af/Am and As/Aw), 4 Arid (BWh, BWk, Fig. 6 shows the areas of the BW (Desert) climate to be located be- BSh, and BSk), 3 Temperate (Ca, Cb, and Cc), 3 Continental (Da, Db, tween 30° N and 30° S. The deserts are easily recognized along the and Dc/Dd) and 1 Polar (E). These are sufficient for deep time zonal areas including North Africa, Arabian Peninsula, and Middle East. paleoclimate studies. In addition, they occupy nearly all the Central–West and A world map using the new classification has been made following some small patches of South Africa and . The BS climate the suggestions of Peel et al. (2007) (Fig. 6). Stations from (GHCN) ver- (Steppe climate) is commonly located at the outer margins of the BW sion 2.0 dataset (Peterson and Vose, 1997) with at least 30 observations climate areas; most are located in Asia and North America. Compared for each month were used in the analysis; this involves 12,396 precipi- to the Köppen climate classification map, the areas of BS climate are ex- tation stations and 4,844 temperature stations. Where available, the panded, especially in Asia. The BS climate is the intermediate between mean annual temperature (MAT), mean annual precipitation (MAP), the arid (BW climate) and humid climates and is a combination of the warm month mean temperature (WMMT), and Köppen aridity index area of these two climate types. The Köppen aridity index (AIKöppen)is (AIKöppen) were calculated for each station. Then a two-dimensional affected by the mixed climates, although we revised the AIKöppen value (latitude and longitude) thin-plate spline interpolation with tension from 13.6 to 10.4. was applied to each parameter onto a 0.1 × 0.1° grid. All the interpola- The areas of the Ca climate (Humid subtropical climate) are mainly tions are performed in ESRI ArcMap version 10.2 using settings of in the southeast of North America, the south of South America, East “weight” = 1 and “points” = 10. The new criteria were then applied Asia, and Southern . The areas of the Cb/Cc (Maritime temper- to the splined parameters. As shown in Fig. 6, the global configuration ate/) are mostly in Northern Western Europe, with of the new world map closely matches the overall pattern of Köppen some isolated areas in southwestern South America and South Africa. system. Note that in the world map of the Köppen climate classification The is undistinguishable in the new world the climate types are combined and reduced to 13. map, because of the distribution of the precipitation cannot be well In the world map of the new climate classification, the areas of the constrained in deep time. The areas shown as Mediterranean climate Af/Am climate (Tropical rainforest climate and Tropical monsoon cli- in Köppen climate classification, such as the area around the Mediterra- mate) commonly straddle the equator, between 5° N and 5° S. The rep- nean Sea, much of California, and parts of Western and South Australia, resentative regions are Southeast Asia, Central and West Africa, and are assigned to other C climates in the new climate classification.

Fig. 3. Correlation between MAP-5Pth from the Köppen climate classification and a) mean annul temperatures, b) mean annual precipitation (MAP), and c) Köppen aridity index (AIKöppen). See Table 1 for detailed explanations of these indicators. L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106 103

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

4.5. Application

In previous studies, it has been assumed that the Paleogene paleoclimate in East Asia was controlled by the planetary wind system with a unique pattern of three latitudinal zones (Guo et al., 2008)(see figures 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 evidence that monsoon or monsoon-like conditions prevailed in East Asia during that period (Quan et al., 2011, 2012b; Wang et al., 2013; Quan et al., Fig. 4. Latitudinal distributions (positive/negative values indicate north/south latitudes) of 2014). This brings into question the planetary wind model and the lati- the mean annual precipitation (MAP) according to the Köppen climate classification. The Af/Am and As/Aw climates are represented by red and green circles. The MAP value of tudinal zonation, especially what has been interpreted as an arid zone. 1800 mm (dotted line) is the boundary between the Af/Am and As/Aw climate types ac- Can the Paleogene of the East Asia interior be considered an arid climate cording to the new paleoclimate classification. Only A climates are plotted in this figure. using the modern climate classification scheme (Quan et al., 2014)? Un- fortunately, verification is impeded by the paucity of data, in terms of both quantity and variety. The new paleoclimate classification with simple parameters is more Compared to the Köppen world map, the areas of the Ca climate are ex- useful. The paleoclimate of East Asia during the Eocene had been recon- panded in North America and Africa, and reduced in Asia in the new structed based on the coexistence approach (CA) using 66 plant assem- world map. blages by Quan et al. (2012a,b) (Table S2). The 66 paleofloras come from The distribution of the D climate () is parallel to 37 fossil sites throughout China, with ages ranging from early to late Eo- latitude. The areas of the Da/Db (Continental climates) are above 40° N cene. The Köppen aridity index (AIKöppen) has been calculated for each to 65° N latitude, within central and northeastern North America, plant assemblage based on the reconstructed MAT and MAP. All of the

Europe, and Asia. The areas of the Dc/Dd (subarctic climate) are gener- Köppen aridity index (AIKöppen) values are larger than the semi-arid ally at latitudes from 50° to 70°N, poleward of the Continental climates. threshold value of 10.4, and there are no obvious discrepancies between In the new map, the areas of the Da climate are often replaced by C cli- the plant assemblages in the humid and arid zones throughout the Eo- mates and areas of the Dc/Dd climates are replaced by the Db climate. cene (Table S2). All of the plant assemblages represent Ca climates, This is because the new classification tends to emphasize warmth for but two of them can also be classified As/Aw and another two classified these climate types. as Cb climate according to their ranges of MAT or WMMT. This pattern is The two major areas with E climate (Ice Cap and Tundra climates) similar to the distribution of climate types in modern China, supporting are Antarctica and Greenland. In addition, most northern islands of the idea of a monsoon or monsoon-like climatology (Quan et al., 2011, and high-latitude also belong to this type. We do not 2012b; Wang et al., 2013; Quan et al., 2014). take altitude specifically into account because of the uncertainties of paleoaltimetry. Some areas with high altitudes (above the snow line) 5. Summary can be assigned to the E climate, such as the Andes, the Himalaya, , and the . Modern climate classifications do not work well in deep time due to In the new classification, 3575 (83.55%) of 4279 locations retain the inherent differences of climates in the past. Climatically sensitive their original major climate types, and 3223 (75.32%) of 4279 locations deposits, paleontological evidence, and modeling have been employed retain their original climate types when classified by subtype (Tables 4 to describe paleoclimates in deep time. However, they have limited

Fig. 5. Latitudinal distributions (positive/negative values indicate north/south latitudes) of warm month mean temperature (WMMT) according to a) the Köppen climate classification and b) the new paleoclimate classification. The a, b, c, and d climates are represented by red, green, yellow, and blue circles. The c and d climates (yellow circles) are combined in the new paleoclimate classification. WMMT values of 21 °C and 15 °C (dotted lines) are the boundaries between adjacent climate types. c) Correlation between the warm month mean temperature

(WMMT) and the maximum monthly temperature (Tmax). The former comes from the new paleoclimate classification and the latter comes from the Köppen climate classification. Only C and D climates are plotted in this figure. 104 L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106

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. applicability, because of our incomplete knowledge of the measure- Eocene, all of East Asia shared the same climate type (Ca: Subtropi- ments of proxies and relatively poor understanding of climate dynamics cal), supporting the monsoon or monsoon-like conditions in East in the past. We propose a new paleoclimate classification based on a Asia during that time, rather than a system controlled by the plane- modification of the Köppen climate classification. It is a simplification tary wind. of the complex modern climate classification using simple quantitative Supplementary data to this article can be found online at http://dx. criteria. The new classification is closely related to but differs from doi.org/10.1016/j.palaeo.2015.11.041. that of Köppen by changing some limits. It can serve to bridge the gap between the modern and deep time climate studies. A world map using the new classification shows the same patterns as the world Acknowledgments map of the Köppen climate classification. Using the new classification,wewereabletoresolvealong- We would thank William W. Hay, who reviewed our manuscript and standing dispute about the climate patterns of East Asia during the gave us many useful comments. Laiming Zhang is supported by a schol- Paleogene. The new classification indicates that during the entire arship by the Chinese Scholarship Council. This study was financially L. Zhang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 443 (2016) 98–106 105

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

A B C D E Total

Koppen's 398 872 1753 1202 54 4279 Paleoclimate 443 1017 1719 1066 34 4279 Unchangedb 372 787 1489 907 20 3575 Percentagec 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 4279 Paleoclimate 152 291 183 196 145 493 1316 367 36 74 693 299 34 4279 Unchanged 130 200 167 104 113 279 1092 332 1 73 551 161 20 3223 Percentage 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%

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