MIT OpenCourseWare http://ocw.mit.edu 12.740 PaleoceanographySpring 2008For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.1PALEOCEANOGRAPHY 12.740 SPRING 2006 Lecture 5 SEA-LEVEL AND CLIMATE CHANGE: CORAL REEFS ON STABLE AND EMERGING ISLANDS CARBON-14, TH-230, AND PALEOMAGNETIC GEOCHRONOLOGY GLOBAL VISCOELASTIC MODELS I. Introduction: shell position vs. initial absolute depth; local vs. eustatic sea level; crustal rebound, continental tilting, and global isostatic adjustment; down-slope movement of shells. II. Recall: ∆ δ18Oforam is a function of ice-volume, isotopic composition of ice masses, local salinity, and temperature; probably more a reflection of ice volume rather than changes in ice isotopic composition, local salinity, or temperature; but how can we be more specific? To some extent, paleo-ecological temperature estimates help support this notion, but they do not appear to be sufficiently reliable to constrain sea-level change with confidence. A. Relation between δ18O and sea-level: ContinentOceanContinentOceanδ180 = 0‰ δ180 = +1.1‰Iceδ180 = -30‰120 mModern Ocean Last Glacial Maximum (18,000 years Before Present)Moδo + Miδi = MtδtIsotope Mass Balance Equation: ∆Mi/ Ao = ∆(Sea level) B. Glaciologists believe that the area of an ice sheet is not linearly related to its volume; instead, V α A1.5 . C. A floating icecap would not significantly change sea level, while it would change the mean isotopic composition of the ocean. D. So it seems that the most reliable constraint on sea level would be provided by reliable paleo-shoreline evidence.2III. "Direct" approach: sample shells on the continental shelf that can be dated by 14C (or peat bogs, trees, which say that sea level was below a certain level). In the late 1950's and 1960's, many marine geologists obtain grab samples from the continental shelf and attempted to construct sea-level curves. (e.g. Curray in the late 50's and early 60's; Milliman and Emery in the late 60's) \__ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~6000 kyr \__ \ \__~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~11000 kyr \ \__ down-slope \__ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~16000 kyr transport? \___ \ | | | | A. These studies generally show lowered sea level towards 18K BUT 1. Large scatter at individual age 2. Was sea-level comparable to that of the present at 30 kyr? (this seems contrary to the the evidence provided by δ18O). Possible explanations: a. Real variations in relative sea level? i. Tectonics ii. Polar shift (surface imbalance) b. Problems with samples? e.g. displacement of shells; erroneous dates (e.g. modern 14C contamination; problems with carbon-14 dating tool? c. Or some very basic misunderstanding/mis-assumption? B. In-place indicators (e.g. coral reefs) seem more reliable than movable materials. Stable coastlines (especially stable oceanic islands) are of coarse preferable from this point of view. I V. 14C dating (Part I). A. 14C: produced by cosmic rays -> neutron -> 14N -> 14C + proton; production rate is proportional to 14N and cosmic ray flux; affected by solar wind and earth's magnetic field. 1. 14C t1/2 = 5730 years. However, by convention, 14C dates are reported relative to a reported 5570 year half-life! (so as not to divide the literature between dates that are not consistent with the currently-accepted half life, and those that are). B. If (14C/12C) in the atmosphere is constant, if the object to be dated obtained its carbon directly from the atmosphere, and if the object to be dated is closed, then dN = -λN dt N/No= e-λt C. The role of contamination of old artifacts by modern carbon: 1 2 3 4 5 6 (halflives) 1/2 1/4 1/8 1/16 1/32 1/64 (14C left) 5700 11400 17100 22800 28500 34200 (yrs) 1. Contamination could occur in the laboratory (blanks); by exchange with groundwater 14C; by3exchange with modern seawater; by bacterial growth on raised deep sea cores. 2. This problem leads to a "practical" limit of 25,000 years for material that was not collected and processed by careful workers. If you don't know who did the work and how careful they are likely to be, distrust all ages older than 25,000 years, and allow a few thousand years potential error at all ages older than about 18,000 years. 3. Therefore: The 30K high stand reconsidered: BS! 4. In the use of continental shelf indicators, 14C to reconstruct sea levels. the problems of the wide range of values at glacial maximum and apparent high stand 30-35 kyrBP can be attributed to modern 14C contamination of much older material. Nonetheless, we would still like to know something about older sea level changes. D. There is a lot more to the 14C story (to be covered later). V. U/230Th dating of corals. A. 238U ----------> 234U --------> 230Th -------> t1/2 : 4.5x109yrs 248,000 yrs 75,200 yrs 235U -------> 231Pa -----> 0.7x109yrs 32,800 yrs B. U (VI) is relatively soluble in seawater (carbonato complexes: e.g. UO22+CO32-; UO22+(CO32-)2), occurs at about 13 nmol/kg (2.3 dpm/kg), and appears to be conservative. 230Th is particle-reactive; i.e. it tends to attach to surfaces rapidly, and so it is removed from seawater on a time scale of ~30 years. Hence it occurs at fairly low concentrations in seawater (<0.1 dpm/100kg at the surface; ~1 dpm/100kg in deep waters). C. Corals incorporate uranium (~2ppm) but very little 230Th. D. Assumptions for 230Th dating 1. Closed system : except for radiodecay and production, no uranium or thorium enters or leaves the object. 2. 230Thinitial = O. 3. Initial 234U/238U activity ratio = 1.15 [this is the ratio observed in seawater; disequilibrium results from alpha-recoil damage in continental rocks and subsequently higher weathering rate of 234U; we assume that this ratio has been stable in the past – but is this justified?] 4. Then: ∂234U∂t=λ238U238U −λ234U234U ∂230Th∂t=λ234U234U −λ230Th230Th where the radionuclides are expressed in concentration units (atoms per g); to convert to activity units (dpm/g), must convert