+LPDOD\DQ7HFWRQLFV:HDWKHULQJ3URFHVVHVDQGWKH6WURQWLXP,VRWRSH5HFRUGLQ0DULQH /LPHVWRQHV $XWKRU V -0(GPRQG 6RXUFH6FLHQFH1HZ6HULHV9RO1R 'HF SS 3XEOLVKHGE\American Association for the Advancement of Science 6WDEOH85/http://www.jstor.org/stable/2882056 . $FFHVVHG Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=aaas. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org Himalayan Tectonics, Weathering Processes, and the Strontium Isotope Record in Marine Limestones J. M. Edmond The time evolution of the isotopic composition of seawater strontium (the ratio of strontium- these variablesare causallyrelated, it has 87 to strontium-86) over the last 500 million years has the form of an asymmetric trough. not proved possible to develop an unam- The values are highest in the Cambrian and Recent (0.7091) and lowest in the Jurassic biguous and specific interpretationof the (0.7067). Superimposed on this trend are a number of smaller oscillations. Consideration isotope recordin terms of explicit mecha- of the geochemical cycle of strontium and the dynamics of weathering shows that only nisms. The consensusis that variationsin Himalayan-style continental collisions can influence the isotope ratio on the scale ob- the intensity of continental weathering served. The contemporary Himalayan orogeny is by far the largest since the late Pre- are important, although this notion has cambrian Pan-African event that produced the high in the Cambrian. not been developed in any detail. The average isotope ratio of Sr in the fluvial flux from the weathering of lime- stones and evaporites over the period of Variationsin the isotopiccomposition of past (4). Observationsindicate that the record varied between about 0.7075 and strontiumin seawater,recorded in marine fluvialflux is dominatedby the weathering 0.7080, a quite narrow range (4). The limestones, are a proxy of the interplayof of platform carbonatesand evaporites of long residencetime of Sr in the limestone mantle processesand crustal weathering. marine origin. There is only a minor reservoiracts strongly to damp the signal Wickmanoriginally proposed that Rb (the contributionfrom the primaryweathering from the relatively high-frequencymarine radioactiveparent of 87Sr, half-life -48 of the refractoryigneous and metamorphic isotopic oscillations (-50 million years). billion years), as an incompatibleelement, basement rocks (5). Hence, the memory Contemporaryoceanic hot springshave a should be progressivelyenriched in conti- effect is dominant. uniform ratio of about 0.7035, which nental crustalrocks over geologic time rel- A synthetic stratigraphiccolumn com- probablyhas not varied over the last 500 ative to Sr (1). Hence, Sr from fluvial posedof marinelimestones has been assem- million years (3). On the basis of the sea inputsto the ocean shouldbecome increas- bled that is continuousover the past 500 floor spreading record of the last 150 ingly radiogenic, leading to a monotonic million years (6). The Sr isotopicrecord of million years, the hydrothermalflux has increasein the isotopiccomposition of Sr in seawaterderived from this column is char- changed slowly and continuously but seawater.Wickman was the first to recog- acterizedby largeoscillations on a varietyof probablyless than ?30% (8), an amount nize that the record of this increase, pre- time scales (Fig. 2). Values of the 87Srto insufficientto resultin any markedchange servedin marinesediments, could be used 86Sr ratio were high in the Cambrian in the isotopic record. Hence, strong fea- for dating, particularlyof limestones and (-0.7091) but declined irregularlyuntil tures in the record most likely reflect evaporites(1). This latteridea has recently the late Jurassic(-0.7067). Several inter- continental processes.Given that the av- been revivedand appliedwith some success mediate-scalefluctuations are superimposed erage Sr flux from limestones has isotopic (2) (Fig. 1). However, the isotopicsystem- on this trend. The continuation of these ratios between the extrema of the oscilla- atics are much more complex than Wick- oscillationsin the Cretaceousand Tertiary man could have realized. was interruptedin the mid-Tertiaryby a The isotopic composition of Sr in sea- steep rise that has continued until the 0.7093 water at any time is determined by the present time (Fig. 1). The contemporary balance between the deep-sea hydrother- seawaterratio (0.7091) is the same as that mal input of nonradiogenicSr of mantle at the height of the Cambrianand is sub- origin (-0.7035) and the more radiogenic stantially higher than at any intervening fluvial flux derived from continental time. These observationsare completelyat weathering(-0.712) (3). The majorsink variancewith the originallinear evolution for Sr is marine carbonaterocks; incorpo- model (1). ration in the carbonatesinvolves no iso- In attempts to explain the features in topic fractionation.The residencetime of the isotope curve, correlationshave been Sr in this limestone reservoir is on the claimedwith many events contained in or (0*0 orderof 150 million years (and in seawater inferredfrom the geologic record (2, 7). 0~~~~~~~~~~~~~~ -3 million years) (3). Variations in the Changes in the fluvial inputs have been continental signal, caused by the dissolu- attributedto variationsin continental gla- 0~~~~~~~~~~~~~~ tion of igneous and metamorphicrocks, ciation, eustatic sea level, atmospheric OR are thereforedamped by the weatheringof CO2 levels, climate, vegetation, and these carbonatesafter exposurecaused by mountain building, individually or in changesin eustaticsea level or by tectonic combination. For time periods before uplift. Because of the weathering of car- about 100 million years ago where the 0.707",'0 20 40 60 80 100 bonates, the fluvial flux retains a "memo- paleomagneticrecord provides little con- Age (millions of years) ry"of isotopic fluctuationsin the geologic straint, changes have been claimed be- Fig. 1. High-resolutionisotope ratio curve for cause of substantialvariations in the oce- The author is in the Department of Earth, Atmospheric, abyssal carbonates over the past 100 million and Planetary Sciences, Massachusetts Institute of anic hydrothermalflux and hence the sea years based on drill core samples [modified Technology, E34-201, Cambridge, MA 02139. floor spreading rate. Because many of from (2)]. The chronometricpotential is clear. 1594 SCIENCE * VOL. 258 * 4 DECEMBER1992 tions and that deviations, such as the Strontium in Rivers global reconnaissance(3, 12). The funda- recent one, extend well above this range, mental resultis that exposedigneous base- the fundamentalvariable is the weather- A large amount of data exists on the Sr ment terrains, in continental shields, do ing yield of radiogenic Sr from old base- systematicsin rivers that characterizethe not yield radiogenicSr in high flux, even ment rocks. To the first order, the prob- relationsamong the Sr flux, the Sr isotope when the rocks are highly alkalic and of lem is reduced to the identification of ratio of the riversand oceans, the geologic greatage. This is trueof both deeplyweath- distinct processes that produce a high events, and weathering environments ered, lateritic soils from basement in the fluvial flux of radiogenic Sr and operate where Sr is releasedfor transport.A num- Tropics,such as that foundin the Guyana, repeatedlyover discrete intervals of geo- ber of local studiesare available(9), along Brazilian,and southernAfrican shields (3, logic time. with some regionalones (10, 11) and one 12), and recently glaciated terrains, as foundin easternCanada (10). Of the other siliceousrocks (12), black shalescan give a high dissolved yield as a result of acid 0.71019]Tertiary Cretaceous Jurassic TdasIc PerrmanPenn] Mbis.sDevonianSilurian Ordoviciancam weathering associatedwith sulfide oxida- tion; basic rocks also weather rapidlybe- cause of the oxidative breakdownof mafic minerals.However, the Sr that is releasedis 0.709 _ not radiogenicbecause of the usualadmix- ture of carbonaterocks in black shales and the mantle affinitiesof volcanic rocks. The only observedexception to this generaliza- 0.708~ - tion is the metamorphiccore complexfrom the centralHimalaya, which influencesthe CD Gangesand Brahmaputrarivers (3, 11, 12). 00 0 Data from a wide varietyof geologic envi- ronments (Fig. 3, A and B) define a uni- form shallowincrease in the isotopic com- 0.70671 position as