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The geochemistry of major and selected trace elements in a forested peat bog, Kalimantan, SE Asia, and its implications for past atmospheric dust deposition

The geochemistry of major and selected trace elements in a forested peat bog, Kalimantan, SE Asia, and its implications for past atmospheric dust deposition
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  PII S0016-7037(02)00834-7 The geochemistry of major and selected trace elements in a forested peat bog, Kalimantan,SE Asia, and its implications for past atmospheric dust deposition D OMINIK  W EISS , 1,2, * W ILLIAM  S HOTYK , 3 J ACK  R IELEY , 4 S USAN  P AGE , 5 M ARLIES  G LOOR , 6 S TEVE  R EESE , 7 andA NTONIO  M ARTINEZ -C ORTIZAS 8 1 Department of Earth Science and Engineering and Department of Environmental Science and Technology, Imperial College of Science,Technology and Medicine, London SW7 2BP, UK 2 Department of Mineralogy, The Natural History Museum, London, UK 3 Institute for Environmental Geochemistry, University of Heidelberg, Heidelberg, Germany 4 School of Geography, University of Nottingham, Nottingham, UK 5 School of Geography, University of Leicester, Leicester, UK 6 Kantonales Labor fuer Umweltschutz, Luzern, Switzerland 7 Physics Institute, University of Bern, Bern, Switzerland 8 Biology Faculty, University of Santiago de Compostela, Santiago de Compostela, Spain(  Received May  2, 2001;  accepted in revised form December   19, 2001) Abstract —Biogeochemical processes in a forested tropical peat deposit and its record of past atmosphericdust deposition were assessed using the vertical distribution of lithophilic and plant essential elements in adated core profile from Borneo, SE Asia. Peat formation started   22,120  14 C yr before present (BP), andCa/Mg mass ratios of the solid peat and very low ash contents indicate a strongly ombrotrophic characterthroughout the deposit, implying that most of the inorganic fraction has been supplied exclusively byatmospheric inputs. Concentration profiles of Mn, Sr, and Ca suggest a very minor influence of chemicaldiagenesis in the underlying sediments. Silicon, Ca, Mg, P, S, and K show a strong and extended zone of enrichment in the top 200 cm of the profile, indicating that biological accumulation mechanisms are muchmore extensive than in temperate peat bogs.In the lower core sections, where the element distribution is dominated solely by past atmosphericdeposition, average Al/Ti ratios are similar to the upper continental crust (UCC), whereas Fe is slightlyenriched and Si is strongly depleted: this condition favors highly weathered tropical soil dust as the maininorganic mineral source. Significant correlation of Al, Fe, Si, S, Ca, and Ti with the lithophilic elements Yand Zr suggests that the distribution of these elements is controlled by sources of atmospheric mineral dust.The Ca/Mg, Ca/K, and Mg/K ratios of the collected rainwater samples are similar to the global average of continental rainwater and suggest a continental character for the site. This is supported by the similarity of theaverage concentration of Br, Mg, Ca, and S to that in temperate continental and maritime bogs in Switzerlandand Scotland.The concentration profiles of Si, Fe, Al, and Ti show distinct peaks within the profile, implying enhanceddust deposition, reduced rates of peat accumulation, or possibly both owing to climatic changes during theHolocene. Enhanced dust deposition between   10,830 and 9060  14 C yr BP is tentatively interpreted as aYounger Dryas–like event with dust fluxes of    10.8 mg/m 2  /yr. The variations in Al/Ti and Fe/Ti profilessuggest that mineral dust sources have been changing constantly during the Holocene, with local sources beingdominant between   7820 and 9500  14 C yr BP and long-range transport (derived most likely from China)being important during the late Pleistocene and early Holocene and from   7820  14 C yr BP to thepresent.  Copyright © 2002 Elsevier Science Ltd  1. INTRODUCTION Peatlands are an essential part of the Earth’s biosphere,accounting for approximately 3% of the total land area. Thus,the role they play in the geochemical cycles of major and traceelements is important on a global scale. While biogeochemicalprocesses of peatlands in temperate regions of the northernhemisphere have been intensively studied (e.g., Damman,1978; Shotyk, 1988; Mandernack et al., 2000), much less is known about tropical peatlands. Tropical peatlands occur inEast and Southeast Asia, the Caribbean, Central and SouthAmerica, and southern Africa, where rainfall and topographyare conducive to poor drainage, permanent waterlogging, andsubstrate acidification. The total area of undeveloped tropicalpeat is estimated to be between 30 and 45 Mha, which isapproximately 12% of the global peatland resource (Immirziand Maltby, 1992). The scarcity of available ecological andgeochemical data on tropical peat, especially in the SoutheastAsian region, has been recognized when assessing its potentialrole and impact on the global carbon budget (Sorensen, 1993;Nichol, 1997).In Southeast Asia, topogenous and ombrogenous are the twomajor categories of peatland. The former are freshwaterswamps that have a restricted distribution along the margins of lowland rivers, and the latter are true peat-forming swampswith water and nutrient supplies derived entirely from aerialdeposition (rain, aerosols, and dust). The surface of theseombrogenous peatlands is usually convex and is positionedabove the limit of wet season river flooding. The rain-fed, * Author to whom correspondence should be addressed( Pergamon Geochimica et Cosmochimica Acta, Vol. 66, No. 13, pp. 2307–2323, 2002Copyright © 2002 Elsevier Science LtdPrinted in the USA. All rights reserved0016-7037/02 $22.00  .00 2307  perched water table is close to or above the peat surfacethroughout the year and fluctuates with the intensity and fre-quency of rainfall (Page et al., 1999).Unlike the  Sphagnum -dominated peat bogs of temperateregions, these peat swamps support large and diverse forestvegetation (Anderson, 1983). Given the known paucity of nutrients in temperate, ombrotrophic peatlands, it has beendifficult to understand how such large, diverse forests can besustained on similar peatlands in tropical regions (Golley et al.,1975; Page et al., 1999), stimulating ecological research effortsin those areas. Geobotanical, ecological, and palynologicalstudies of tropical, ombrotrophic peats (water and nutrients aresupplied only by aerial deposition) in Southeast Asia have beenconducted mainly in Sarawak and Kalimantan (Anderson andMuller, 1975; Sieffermann et al., 1988; Moore and Hilbert,1992; Moore et al., 1996). However, geochemical studies of ombrotrophic bogs in these regions have been very limited andhave focused on the peat as a possible analog for low sulfur orlow ash coal (Cameron et al., 1989; Cobb and Blaine, 1993;Neuzil et al., 1993; Moore et al., 1996; Wuest and Bustin,2001). These studies have shown that the minerals entering thedeposits were from allochtonous sources of dry and wet dep-osition. Ash yields and sulfur content were low throughoutmost of the peat deposits (  10% and 1%, respectively), and Si,Al, and Fe were the abundant inorganic constituents (Neuzil etal., 1993; Moore et al., 1996). Despite this previous work, adetailed biogeochemical study linking element distribution tosource assessment in tropical peats has not yet been undertaken.Peat bogs can also provide reliable records of the changingrates of atmospheric dust deposition and aerosol composition(Shotyk et al., 2001). As continental geochemical archives,they have the unique advantage of a wide global distributioncompared to ice cores (the only two archives that record ex-clusively atmospheric deposition). Even at pH 4 in organic-richbog waters, atmospherically deposited, fine-grained mineralparticles, including various silicate materials such as quartz andfeldspar, seem not to weather further after deposition (Stein-mann and Shotyk, 1997a, 1997b; Wuest and Bustin, in press).This is possibly because (a) protective organic coatings fromdissolved organic matter are formed on the surfaces of themineral particles (Steinmann and Shotyk, 1997a), (b) the sol-ubility of Si in equilibrium with quartz is at its lowest in acidicsolution (Krauskopf, 1956; Wilding et al., 1979; Rimstidt and Barnes, 1980; Stumm and Morgan, 1996), and (c) the soilminerals entering a bog are already highly weathered. A com-plete record derived from a peat core in Switzerland has sub-sequently provided new insights about the relationship betweenchanging climate, long-range transport, and the chemical com-position of mineral aerosols derived from weathering for theentire Holocene (Weiss et al., 1997; Shotyk et al., 1998; Shotyk  et al., 2001). Enhanced fluxes of dust occurred during theYounger Dryas between 11,440 and 9320  14 C yr before present(BP), and a shift toward significantly more radiogenic lead wasobserved at 8520  14 C yr BP, coinciding with the cooling eventfrom  9000 to 7000 calendar years ago, a prominent Holoceneclimatic episode found in the Greenland ice core (Alley et al.,1997; Blunier and Brook, 2001). Similar records of atmo-spheric dust deposition are badly needed for the tropical re-gions (Peteet, 1995; Seltzer, 2001) but are at present limited to a few studies using ice cores from the Andes and Tibet (e.g.,Thompson et al., 1997, 1998). To assess the dominant biogeochemical processes in a trop-ical, ombrotrophic peat deposit and to study the dust depositionin the Southeast Asian region, a core of the interior peatlands of Central Kalimantan representing   22,120 yr of peat accumu-lation has been investigated. We aimed to (a) quantify thevertical distribution of the major and trace elements Si, Al, Fe,Ti, Ca, Mg, K, Na, S, and P in the peat profile; (b) assess thetrophic status of the deposit and the biologic enrichment in thesurface layers; (c) evaluate the sources of major elements(marine vs. terrestrial); and (d) assess the changing rates of dustfluxes and sources. 2. MATERIALS AND METHODS2.1. Site Description The peat deposit of Palangka Raya is located in the upper catchmentof the Sungai (river) Sebangau, Central Kalimantan, Indonesia (Fig. 1).The Sebangau catchment (5000 km 2 ) consists of a large, continuousarea of relatively undisturbed peatland, much of which is still coveredby forest (Sieffermann et al., 1988). The peat is strongly domed, with  12 m of peat accumulation in the center (Shepherd et al., 1997; Pageet al., 1999). The core for this study was taken approximately 7.5 kmfrom the dry season riverbank (Fig. 1). The dominant forest type at thislocation is a low pole forest (Shepherd et al., 1997; Page et al., 1999).Pneumatophores are abundant, and a dense mat of tree roots exists inthe surface peat. Kalimantan lies within the intertropical convergencezone (ITCZ); thus, wet deposition is significantly more important thandry deposition. Rainfall generally exceeds 2500 mm/yr (Page et al.,1999), and even in the dry season, the minimum rainfall exceedsevapotranspiration in the peat swamp forests (Morley, 1981). 2.2. Sample Collection and Preparation The core (denoted as SA6.5) was taken in August and September1995. From 0 to 30 cm deep, a 15  20 cm monolith was cut out froma hummock using a stainless steel knife and divided subsequently into3-cm increments and packed into plastic bags. Much of the monolithconsisted of raw forest humus with pieces of wood and living roots. Acontinuous core was then obtained using a 5.0  50.0 cm Belarus peatsampler. The 50-cm core sections were cut into two 20-cm sections andone 10-cm section in the field and packaged in plastic bags. Twoadjacent 10-cm sections were combined. The first 80 cm were verywatery and consisted of poorly to moderately decomposed organicmaterial, mainly peaty humus with some wood. The sections from 80to 840 cm consisted of well-decomposed to very well-decomposed peatwith wood and herbaceous material. The sediment-peat interface oc-curred at 840 cm, and the lithogenic sediment started at 940 cm. Thezones were identified visually by a change in color and by the amountof organic matter. Samples were brought to the laboratory and weighed,and the pore water was squeezed out by hand using plastic gloves.Afterward, the peats were dried at 105°C in acid-cleaned Teflon bowls,macerated in a centrifugal mill equipped with a Ti rotor and a 0.25-mmsieve in a class 100, laminar flow clean air cabinet and stored inacid-washed polyethylene beakers with screw caps. Possible contami-nation from the mill was monitored using certified plant material. 2.3. Chemical Analysis of Peats and Statistical Data Treatment To determine the ash content,  1.5 g of previously dried, milled, andhomogenized peat were placed in a preheated, acid-washed porcelaincrucible and dried again at 105°C for 4 h. After cooling in a desiccator,dry weight was determined to 1 mg. Ashing was accomplished byheating at 550°C overnight. After cooling in the desiccator for 15 min,the crucible was weighed again and the ash content weight calculated.Titanium, Ca, K, Zr, and Y were analyzed by inductively coupledplasma mass spectroscopy (ICP-MS) (ELAN 5000). Silicon, Al, Fe,Mg, Mn, Na, P, and S were analyzed by inductively coupled plasma2308 D. Weiss et al.  optical emission spectroscopy (Optima 3000). About 250 mg of thedried sample were digested using 4 mL concentrated HNO 3  (69%), 3mL concentrated H 2 O 2  (30%), 0.4 mL concentrated HF (40%), andlow-pressure microwave heating (Weiss et al., 1999b). All acids wereof suprapure (Merck) quality. Analytical precision was assessed bymultiple analysis of selected peat or standard reference material sam-ples and was   38% for concentrations   10 ng/g,   14% (10 ng/g),  10% (30 ng/g),   6% (100 ng/g),   5% (200 ng/g), and   5% (  5  g/g). Analytical accuracy was determined using the certified plantmaterials Apple Leaves (National Bureau of Standards [NBS] 1515),Peach Leaves (NBS 1547), and Pine Needle (NBS 1575) and waswithin   12% for all elements. The detection limits for all elementswere well below the lowest measured concentrations in samples. Theelements Al, Ca, K, Mg, P, S, and Si were also analyzed by wave-length-dispersive X-ray fluorescence spectroscopy at the GeologicalInstitute, Ukrainian Academy of Science, and Br, Sr, and Rb weremeasured using energy-dispersive miniprobe multielement analyzerX-ray fluorescence (Weiss et al., 1998). Both measurements agreedwithin  12% for all elements.To estimate the degree of interrelation between elements or ele-ment/Ti ratios, the correlation coefficient  r   was calculated from theratio of the covariance of the two elements (COV  j , k  ) to the product of their standard deviations ( s  j ,  s k  ) using the relationship of   r   j , k   COV  j , k   /  s  j s k  . Consequently, a  t   test (two tailed) was used as a parametricalstatistical test to assess a 90% level of confidence for the correlation(Davis, 1986). 2.4. Normalization of the Chemical Profiles to Ti The resistance of Ti-bearing minerals such as rutile (TiO 2 ) to chem-ical weathering in acidic solutions allows Ti to be used as a conserva-tive reference element in soils and sediments (Nesbitt and Markovics,Fig. 1. Map showing the approximate location (2°18'22“ S, 113°54'36” E) of the sampling site (core SA6.5) in the SungaiSebangau catchment on the island of Kalimantan (Borneo). River names are given in italics.2309Peat bog geochemistry and implications for atmospheric dust deposition in SE Asia  1997). To verify the conservative character of Ti under tropical con-ditions characteristic of the region being studied, Y and Zr, two otherlithophile elements well known for their stability against weathering(Nesbitt and Markovics, 1997; Young and Nesbitt, 1998), were also measured. The correlation between Ti and Y was significant (Y   0.0021 Ti  0.0221,  r  2  0.893,  n  57), as was that between Ti andZr (Zr  0.019 Ti  0.040,  r  2  0.966,  n  57) (Fig. 2). The reasonsfor choosing Ti over the other conservative elements were the higherconcentrations of Ti in the peat samples and the lower detection limitsobtained for this element using ICP-MS. Unfortunately, Ti and someother elements were not measured in the deepest layers because theirconcentrations were outside of the calibration range using ICP-MS. 2.5. Chronology of Peat Accumulation Age dates were obtained by  14 C decay counting. The dried, pow-dered samples were pretreated with HCl-NaOH-HCl and then burned ina quartz glass tube in oxygen. The resulting CO 2  was purified, capturedwith liquid nitrogen, and subsequently reduced with hydrogen using Rucatalyst at 300°C into methane. The methane was counted in theunderground laboratories (Physics Institute, University of Bern) for70 h. All results are reported here as conventional radiocarbon agedates ( 14 C yr BP) and as calibrated years (cal yr BP). Calibrated ageswere calculated using CALIB 4.1.2 and are reported as intercepts or therange of intercepts (Stuiver and Reimer, 1993). 3. RESULTS 3.1. Age Dating The age dates are given as conventional radiocarbon years,and the calibrated intercepts are shown in Table 1. The peataccumulation rates and their implications for climate will bediscussed in detail in a separate paper (Rieley et al., in prepa-ration). Here, we summarize briefly the main characteristicsrelevant to this paper. Peat accumulation started  22,120  14 Cyr BP, dated at the peat-sediment interface at 840 cm. This isthe oldest age reported for any SE Asian ombrotrophic peatdeposit, but it is consistent with a recently published radiocar-bon age from a freshwater swamp forest on Singapore Island,suggesting that peat accumulation began   23,000  14 C y BP(Taylor et al., in press), following the stabilization of sea levelafter glacial retreat (Geyh et al., 1979; Sorensen, 1993; Seltzer, 2001).Within the entire profile, three sections have radiocarbon ageinversions in which older dated peat overlies younger peat(Table 1). However, two of these inversions are not significantfor the interpretation of the profile. One inversion occurs in thesediment-peat interface (between 960 and 900 cm), and theother has almost overlapping radiocarbon ages, suggesting thatthe samples of this section belong to the same period (samplesSA6.5-28, and SA6.5-31, Table 1). Regarding the third section,between 750 and 690 cm, the radiocarbon age of the sampleabove the inversion (  10,320  14 C yr BP, sample SA6.5-42) isindistinguishable from the radiocarbon age of the lowest sec-tion sample (  10,250  14 C yr BP, sample SA6.5-44). Thus, onlytwo samples have unexpected younger ages (7690 and 7090 14 C yr BP, samples SA6.5-44 and SA6.5-45). These samplescorrelate with significantly lower concentrations of the litho-philic elements. At present, we cannot fully account for thesetwo significant younger ages. Contamination with “young”  14 Cin the laboratory can be ruled out because separate acceleratormass spectrometry dating of dust remaining in the srcinal jarsyielded identical ages within error (unpublished results). Also,mistakes during sample handling or other chain-of-custodyerrors can be excluded. Transport of humus down in the profileseems unlikely because the samples used for age dating werecomparatively large (i.e., 5 to 8 g of dry material). The twomost likely explanations for the  14 C inversions are either pen-etration of tree roots into deeper, older peat layers, and thesubsequent introduction of large quantities of younger C,and/or tree-fall, which would introduce older peat on top of younger material.Despite these inversions, however, there are distinct periods Fig. 2. Scatter diagram of Ti vs. Zr and Y in the entire core of SA6.5. Titanium shows statistically significant correlationswith Y (Y  0.0021 Ti  0.0221,  r  2  0.893,  n  57) and Zr (Zr  0.019 Ti  0.040,  r  2  0.966,  n  57)2310 D. Weiss et al.  for which varying rates of peat accumulation are clearly iden-tifiable. Peat accumulation (calculated using incremental accu-mulation of total dry peat mass and radiocarbon ages) was verylow in the late Pleistocene (between  22,620 and  10,830  14 Cyr BP), with only 0.003 g/cm 2  /yr. This suggests that the bogsrcinated from an organic deposit at the end of the Pleistocene,in a time interval approaching that of the Last Glacial Maxi-mum (LGM). Bulk density and ash contents are higher thanthose of the layers above, and the bulk density is comparableonly with the upper rooted zone of the profile (Fig. 3), indicat-ing possible compaction of the peat. The peat growth must havebeen affected dramatically by the low temperatures and re-duced humidity characteristic of the LGM (Barnola et al., 1988;Yung et al., 1996; Petit et al., 1999; Weyhenmeyer et al., 2000;Seltzer, 2001). During the early Holocene (between   10,830and   10,320  14 C yr BP), peat accumulation increased nearlythreefold to 0.008 g/cm 2  /yr. Peat accumulation rates decreasedduring the mid-Holocene until   6070  14 C yr BP to 0.006g/cm 2  /yr and decreased further in the late Holocene between  6070  14 C yr BP and the present day to 0.001 g/cm 2  /yr. 3.2. Bulk Density, Ash Content, and pH Dry bulk density of the peat core ranges from 0.02 to 0.21g/cm 3 (Fig. 3). The increase with depth in the uppermost 150cm reflects the transition from living plant material (roots)through dead plant material to poorly decomposed peat. Below200 cm, the density decreases to  0.05 g/cm 3 , with five peakscharacterized by densities above 0.07 g/cm 3 (Fig. 3).Ash concentrations are higher in the top of the core (  1.0%)and very low in the middle part of the profile between 200 and840 cm (0.33 to 0.65%), and they increase in the bottomtransition zone between peat and mineral sediments (from 3.8to 5.2%). The sediment has ash concentrations above 6%. Thecore has slightly lower ash concentrations between 200 and 540cm (0.33  0.13%,  n  18) compared to the zone from 560 to840 cm (0.65    0.23%,  n    15). Major ash peaks and/orintervals above 0.7% are found at 120 and 560 cm, between640 and 680 cm, and between 740 and 780 cm.The pH of the moist peat averages 3.2    0.4 and is fairlyconstant except at depths with higher ash (Fig. 3). The pHincreases within the peat-sediment interface and reaches 4.5 inthe sediment at 970 cm. Values in the corresponding pore water(not shown) are slightly higher than in the solid peat itself butreveal a similar profile. The fairly constant pH in pore waterand in peat suggests that dissolution of mineral matter isinsufficient to neutralize the acidity generated by the decom-position of organic matter (Shotyk, 1988; Neuzil et al., 1993). 3.3. Silicon, Al, Fe, and Ti The refractory lithophilic elements Si, Al, Fe, and Ti showremarkably similar profiles (Fig. 4). Concentrations increasewith depth within the top 150 cm to form a first peak between Table 1. Radiocarbon age dates of core SA6.5 peat samples.Sample ID Lab. ID Sample type Depth (cm)Radiocarbon age( 14 C yr BP) Calibrated years (cal yr BP) CommentsSA6.5-10 B-7444 Peat 27-30 ModernSA6.5-13 B-7445 90-110 170  60 275, 174, 148, 114SA6.5-14 B-7582 110-130 540  60 542SA6.5-15 B-6917 130-150 1450  40 1331, 1316, 1315SA6.5-16 B-7583 150-170 2230  90 2306, 2234, 2207, 2191, 2183SA6.5-17 B-7446 170-190 3960  80 4417SA6.5-18 B-7584 190-210 4670  80 5447, 5408, 5326SA6.5-19 B-7585 210-230 5790  80 6634, 6583, 6570SA6.5-20 B-6918 231-250 6070  40 6988, 6806SA6.5-21 B-7586 250-270 6880  90 7681SA6.5-22 B-7447 270-290 7090  60 7935, 7891, 7875SA6.5-25 B-7448 330-350 7820  50 8593SA6.5-28 B-7449 390-410 8280  100 9395, 9390, 9366, 9363, 9279SA6.5-31 B-7450 450-470 8120  60 9027 Age inversionSA6.5-38 B-7451 570-590 8540  100 9530SA6.5-40 B-6919 610-630 9060  100 10,358, 10,177SA6.5-41 B-7452 630-650 9470  50 10,691SA6.5-42 B-6920 650-670 10,320  50 12,344, 12,288, 12,234, 12,122, 11,940SA6.5-44 B-7587 690-710 7690  90 8424 Age inversionSA6.5-45 B-6921 710-730 7090  90 7973, 7935, 7891, 7876, 7792 Age inversionSA6.5-46 B-7453 730-750 10,250  110 11,952 Age inversionSA6.5-47 B-7454 750-770 10,440  110 12,596, 12,506, 12,353SA6.5-48 B-7588 770-790 10,830  120 12,899SA6.5-52 B-7455 Peat/sediment 840-860 22,120  320 —SA6.5-53 B-7590 860-880 22,270  130 —SA6.5-54 B-7456 880-900 22,620  310 —SA6.5-55 B-7591 900-920 22,360  130 — Age inversionSA6.5-56 B-6922 920-940 18,280  340 — Age inversionSA6.5-57 B-7592 Sediment 940-960 20,350  130 — Age inversionThe age dates are reported here as conventional  14 C years ( 14 C yr before present [BP]) and as calibrated years BP (cal yr BP). Calibrated ages werecalculated using CALIB REV. and are reported as intercepts or the range of intercepts (see text for details).2311Peat bog geochemistry and implications for atmospheric dust deposition in SE Asia
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