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A Return to Phanerozoic Average Sea Level?

  1. Sep 6, 2009 #1
    Might we be near the end of a large cyclical regression; not so different from Permian end regression? Statistically might it not seem more likely that we are due to revert at least towards an average of sea level for overall Phanerozoic http://en.wikipedia.org/wiki/Phanerozoic" [Broken], or even a further transgression? Might this be a natural underlying trend, independent of any superimposed anthropomorphic effect? Might we be headed for an Eocene/Paleocene world?

    30 million years hence, what might our 'present' stratum (pl: strata) of say 10 million years look like? Might there be any evidence of mankind? For example, if we occupy the middle of such strata, then plus or minus 5 million years. For the past 5 million years, there was no effect. For the future 5 million years, transgression and Yellowstone's Western and Midwest repeated ash fallout would seem to reveal nature's dominant hand. For 200 meter elevation of sea level to less than Cretaceous peak, most of southern U.S. would be inundated, and likewise for eastern coast. The Seaway would flood and enlarge Great Lakes into an inland sea. All coastal cities, and inland lake ports would become reefs initially, and then dissolution. Humanity would would once again be on the move. Therefore, might there be no evidence of mankind's handiwork in such strata (stratum); not even hard plastic cherts? So from a geological perspective, mankind's impact on the environment might be quite negligible, in comparison to nature's broader, deeper, more sustainable ways. Does our myopia greatly underestimate nature's scope and impact, in comparison to that of mankind's? http://en.wikipedia.org/wiki/Sea_level" [Broken]
     
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  3. Sep 6, 2009 #2
    Quite a lot of speculations no? Little use to philosophize about the next ten million years.

    Apart from that, there are a few things to say about that sea level graph in wiki, a better source here.

    http://www.ncdc.noaa.gov/paleo/pubs/ipcc2007/fig68.jpg

    First of all, one could wonder where all that water came from, after all, 125 meters sea level rise is about equivalent to two Antarctic ice sheets, which is still there.

    Of course it is long believed that http://stommel.tamu.edu/~baum/paleoveg/veg-adams-big.gif challenges that rather convincingly (fig 1):

    34ryedk.jpg

    So it would be interesting to see where all that water came from.

    A second problem is the timing. It has been argued here that rather extensive northern hemisphere warming started much earlier, some 17-18 thousand years ago while the sea level rise really started to accellerate around 15 thousand years ago, with the so called Melt Water Pulse 1A.

    I made a thread about that a long time ago, demonstrating that with the current standing of the research neither the Antarctic nor the North American ice sheets could have been the cause of that accelerated sea level rise.

    Maybe we are looking at something different.
     
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  4. Sep 6, 2009 #3
    In addition to that there are the benthic foraminifera isotope series, which swing in correlation with the isotopes in the ice sheets. However because the oceanic bottom water is rather constant, it is believed that these (oxygen) isotopes are a proxy for sealevel. Due to fractination/destillation effects, when more water, with light isotope ratios, is on the ice sheets, the remaining isotope ratio in the oceans is enriched.

    The most recent compilation of those isotopes is LR04, with the data here. It can be seen from that data that the last isotope depletion event (thought to be sea level rise) started at 18 thousand years ago, hence well before the sea level rise graph. In addition to that one has also to take the oceanic inertia into consideration, where it takes a very long time before the light melt waters at the surface, reach the bottom dwelling foraminifera all over the Altantic and Pacific.
     
  5. Sep 6, 2009 #4

    Xnn

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    Andre;

    Just goes to show how paleo data is often not consistent.
     
  6. Sep 7, 2009 #5

    Xnn

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    Sorry, but that post was not at all convincing.

    There is little reason why the Greenland ice cap should warm at the same time as the Cordilleran or southern edge of the Laurentide ice sheets. They were separated by thousands of miles. The Laurentide ice sheet at one time reached all the way to Kansas and nearly to Missouri!

    Also, there was no mention of the Patagonia ice sheet or the combined effects of melting along the warmest areas of all the ice sheets that existed at the time.

    The problem with these ancient ice sheets is that there is no way to obtain ice cores for measuring when and where the melting occurred. Extrapolating data that was collected from such a distance is similar to using New York City weather data to draw conclusions about conditions in Los Angeles. It should be little surprise that things don't line up.
     
  7. Sep 7, 2009 #6
    But not during the last glacial maximum, that's why it was called Wisconsin, it did not reach beyond northern Indiana Northern Ohio.

    But this is all moot because it's not me, claiming that it the North American ice sheets were not the source of meltwater pulse 1A, it was ....

    Weaver A.J. et al (2003) Meltwater Pulse 1A from Antarctica as a Trigger of the Bølling-Allerød Warm Interval 14 March 2003 Vol 299 Science pp1710 - 1713



    Also we're talking about 20 meters in 500 years, the equivalent of three Greenland ice sheets. Can we store that on Patagonia?
     
    Last edited: Sep 7, 2009
  8. Sep 7, 2009 #7
    Update, here is the most recent ice sheet - isotope - volume modelling for the last glacial maximum, that I'm aware of.

    Bintanja, Oerlemans 2002; Global ice volume variations through the last glacial cycle simulated by a 3-D ice-dynamical model, Quaternary International Volumes 95-96, September 2002, Pages 11-23

    In reality the last Eurasian Weichselian ice sheet of the last glacial maximum was considerable smaller than the Laurentide and Cordillerian ice sheet and ice fields.

    see previous post:
    34ryedk.jpg
     
    Last edited: Sep 7, 2009
  9. Sep 7, 2009 #8

    Xnn

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    Thanks for the clarification Andre;

    I initially thought you were implying that neither the Antarctic or North American ice sheets could have contributed to a rapid sea level rise.

    True; the Patagonian ice sheet was small (1 or 2 meters of sea level) however the North American Sheets were huge; about 100 meters. There were also ice sheets in Asia and Australia and some large ones in mountains near the equator (Java?). So, at that time, there was a lot of ice along the fringes in both hemispheres and most of it was a long way from Greenland.

    It'd be interesting to see just how much ice the different sheets had in meters of sea level, but that'd be a research project in itself. There were also massive melt water lakes behind ice dams at the time. Not much warming in Greenland would be necessary to unleash a meltwater pulse from Lake Missoula for example.

    http://en.wikipedia.org/wiki/Glacial_Lake_Missoula

    My subscriptiont to Science is currently broken, so I can't read and comment on the article, but will attempt to do so in the future. However, it looks like the paper concluded that that particular meltwater pulse came from Antarctica.
     
  10. Sep 7, 2009 #9
    Xnn, again we are talking about the last glacial maximum, no ice on Siberia, the Himalayan ice fields are currently under scrutiny. I'll come back to that later, but see my previous post on the problem between the assumptions of Bintanja's model and the results of geologic surveys.
     
  11. Sep 7, 2009 #10

    Xnn

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    The Bintanja model is very bad; and I believe he has widely acknowledged it. As I recall, he did not take into account that ice sheets will alter weather patterns and of course the bigger ice sheets become the larger the impact. Of course, to do things like this properly cost time and money.
     
  12. Sep 7, 2009 #11
    Bad or not you'd still have to find the equivalent of almost two complete Antarctic ice sheets to melt during the last glacial maximum to account for the ~125 meters sea level rise.
     
  13. Sep 7, 2009 #12

    Xnn

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    Andre;

    Oh come on now!

    Not even a single ice cube?
     
  14. Sep 7, 2009 #13
  15. Sep 7, 2009 #14

    Xnn

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    Siberia is a huge area with plenty of room for Mammoths and Ice sheets to co-exist.
    Naturally Mammoths tended to move around in search of food. Of course, they never found any on ice sheets, but instead browsed in nearby areas that were seasonally frozen. During the winter, they'd head south towards warmer pastures. Don't know how far a Mammoth can walk, but on a steppe area, it could be hundreds of miles. On top of that, general circulation and precipitation patterns during glacial maximums were not the same as today. This is particularly important in what are now arid areas.

    http://www3.interscience.wiley.com/journal/119936887/abstract?CRETRY=1&SRETRY=0

    There were also large ice-dammed lakes. So yes; no surprise, there were areas of Siberia that were not constantly frozen, and thus provided excellent habitat for Mammoths. In fact, a lot of people wonder why they died out. Hunting by human dressed in fur was a factor, but maybe the drying out of the landscape was too.

    http://www3.interscience.wiley.com/journal/119936887/abstract?CRETRY=1&SRETRY=0
     
  16. Sep 7, 2009 #15
    Maybe it would help if you read the thread and compare for instance the dates of the references.

    And if "mammoth" gives too much association with howling blizzards and ice sheets, think horses and antelopes instead.
     
  17. Sep 7, 2009 #16

    Xnn

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    Zebras are like horses, one lives in the heat of Africa while the other adapted to colder climates. Both are able to travel great distance, migrating north and south. Genetically, the same family, but wide spread adaptation.

    Likewise, Antelopes and Musk Ox. Similar creatures that have adapted to very different climates.

    With a shaggy coat of fur, even primates from sunny Africa could live in the Tundra.
    Guess I don't see a problem with various creatures adapting to cold climates.

    Keep in mind, that even during the height of the ice age, there were still summers every year and 12 to 14,000 years ago, the sun was brighter during northern hemisphere summers than it is now.
     
  18. Sep 8, 2009 #17

    sylas

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    I agree with Andre in #2 that the original post #1 was vague and speculative. But you've seeded an interesting discussion, zankaon.

    I'm taking [post=2336051]message #2[/post] by Andre as the real origin of the thread. This is where we get genuine substance and linkage to ideas in the scientific literature, which makes a basis for useful further discussion.

    On the other hand, a lot of the subsequent discussion seems to be rather beside the point. Specifically, Andre appears to be saying that there's some kind of problem finding enough ice to account for sea level rises since the last glacial maximum. (Correct me if I am wrong.) However, no reference anywhere that I can see makes such a strange argument. It seems to be exclusively Andre's concern, and is backed up by alleged evidence conflicts that don't actually make the point at all, that I can see.

    There are certainly open questions in science concerning extent and volume of the various different ice sheets and the timing of their advance and retreat, but this is about different ideas for ice distribution; not concerns with finding enough ice in total. Note especially that volume is different from extent. The height of ice sheets is much harder to constrain directly from available evidence than the extent.

    Ice sheets and sea levels at Last Glacial Maximum

    I recommend http://www.ingentaconnect.com/content/els/02773791/2002/00000021/00000001 [Broken] as a good general purpose source. It is a special issue with many papers all focused on ice sheets and sea level for the Last Glacial Maximum. The introduction paper is:
    • Clark, P.U. and Mix, A.C. (2002) http://dx.doi.org/10.1016/S0277-3791(01)00118-4 [Broken], in Quaternary Science Reviews, Vol 21, Iss 1-3, Jan 2002, pp 1-7, doi:10.1016/S0277-3791(01)00118-4
    Abstract: This paper outlines the general issues regarding ice sheets and sea level of the Last Glacial Maximum (LGM), which formed the basis of an EPILOG Project workshop. Papers in this special issue of Quaternary Science Reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent, records of sea level change, ice sheet modelling, geophysical models of glacial isostatic adjustment, and geochemical proxies of ice volume. This new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies, suggesting a net decrease in eustatic sea level at the LGM ranging from 120 to 135 m.
    A summary of different results is given as Table 1 of this paper, giving estimates of excess ice-equivalent sea level in meters for LGM ice sheets. I've used the "code" tag as a way to give the aligned table, and have removed the footnotes, and two columns which only give a total without any allocations to ice sheets. Columns give max and min for the CLIMAP estimates made in 1981 that this workshop was revisiting, then three columns for new estimates taken from papers in this same special issue. There are max and min for ice sheet modeling based estimates, and another for estimates by Peltier. (The footnotes clarify that Peltier's 6m estimate for Greenland is known to be too high, and that he has since revised his estimates to about 3m, in a subsequent paper.)

    Table 1 of Clark and Mix (2002) for estimates of excess ice-equivalent sea level in meters for LGM ice sheets:
    Code (Text):

    Ice Sheet      CLIMAP Min CLIMAP Max  Ice sheet Min  Ice sheet Max  Peltier
    Antarctica      24.5        24.5        14.0           21.0          17.6
    North America   77.0        92.0        82.4           82.4          64.3
    Greenland        1.0         6.5         2.0            3.0           6.0
    Scand/Barents   20.0        34.0        13.8           18.0          25.5
    All others       5.0         6.0         6.0            6.0
    Total          127.5       163.0       118.2          130.4         113.5
    The important take home message for discussion in this thread is that the major part of total sea level rise is easily from North America. The Eurasian sheets are given here as "Scand/Barents". These numbers confirm the major point being made by Andre about the limited contributions from Eurasia, but without making the claim that there's some kind of associated problem with finding enough water altogether.

    North American Ice Sheets

    Much of the thread has focused on Siberia and Asia; but actually North America stands out as much more significant in total for sea level changes. The following paper from the special issue sets out a large range of possible values in some detail.
    Abstract:The areal extent of the last glacial maximum (LGM) ice sheets is well known in North America, but there is no direct geological proxy for ice sheet thickness or volume. Uncertainties associated with glaciological and geophysical reconstructions give widely varying estimates of North American Ice Sheet (NAIS) volume at LGM. In an effort to quantify the uncertainty associated with glaciological reconstructions, we conducted a suite of 190 numerical simulations of the last glacial cycle in North America, prescribing different climatic, mass balance, glaciologic, and isostatic treatments for the least constrained model variables. LGM ice sheet reconstructions were evaluated using the well-established geologic record of ice sheet area and southern extent at LGM (Dyke and Prest (1987)). These constraints give a subset of 33 simulations that produce reasonable LGM ice cover in North America. Ice sheet dispositions and the associated parameter settings in this subset of tests provide insight into the plausible range of NAIS thickness, form, and mass balance regime at LGM. Ice volume in this subset of tests spans a range of 28.5–38.9×1015 m3 at LGM, with a predominant cluster at 32–36×1015 m3. Taking floating ice and displaced continental water into account, this corresponds to 69–94 m eustatic sea level (msl). More than 75% of the accepted tests fall in the range 78–88 msl. We argue that this is a plausible estimate of the volume of water locked up in the NAIS at LGM.
    By comparison, the total ice volume in the present Antarctic ice sheet is about 30×1015 m3, and if entirely melted would raise sea levels by about 70 meters. That is, at LGM there was more ice in North America than there is now in the whole Antarctic.

    Eurasian Ice Sheets

    Most of the discussion in this thread has focused on the Eurasian sheets. There have been various substantially different proposals given in the scientific literature over the years, but by and large the best supported views at present seem to be about the same as Andre's account of the Eurasian situation. So Andre seems to be right in the mainstream as far as Eurasian ice sheets are concerned, as far as I can tell.

    (I'm guessing that the line "no ice in Siberia" in [post=2337716]msg #9[/post] was hyperbole, but correct me if I am wrong about that.)

    What the relevant science says is that there were large ice sheets in Eurasia; but not nearly as large as in North America. At the LGM these sheets only encroached upon a comparatively small part of Siberia. This does not represent any problem for finding ice for sea level rises.

    An aside on mammoths, or flora and fauna generally

    All the stuff on mammoths that has been raised recently here and in another thread has left me a bit puzzled, and now I think I know why. It's presented as if it was some kind of dramatic challenge, but the claims made for ice extent and mammoths in particular seems to be pretty standard and represents the view of most scientists working on the subject. Xnn's comments (#14 and #16) seem to indicate the same puzzlement I have had. Andre's material on the range of mammoths and the extent of ice is simply not a dramatic challenge to the mainstream at all... it IS the mainstream.

    I am not referring here to Andre's claims from the other thread that mammoths or other observations of flora and fauna require substantially higher temperatures than are conventionally proposed on the basis of standard temperature proxies. I am not aware of anyone publishing such a strange notion; the conventional view seems to be as set out in the reference Andre provided for Eurasian climate in msg #2. Andre recommended a link he marked as "more recent research". The full citation is

    This gives the conventional view point. From the abstract:
    ... Palaeoclimatic and palaeoenvironmental conditions for the time prior to, during, and after the LGM have been reconstructed for the non-glaciated areas around the LGM ice sheet with the use of faunal and vegetation records, permafrost, eolian sediments, alluvial deposits and other evidences. The changing environment, from interstadial conditions around 30 ka BP to a much colder and drier environment at the culmination of the LGM at 20–15 ka BP, and the beginning of warming around 15 ka BP have been elaborated from the field data, which fits well with the modelling results.
    In other words: for most of Siberia at the LGM you have very cold temperatures, but not an ice sheet. This is perfectly consistent with the presence of flora and fauna. There's a lot of discussion on this in the reference; here is a relevant sample from page 1340:
    It should be noted, however, that even during the peak of the LGM (18–20 ka BP) mammoth still inhabited the whole area of the Laptev Shelf Land, including the present northernmost islands and Severnaya Zemlya. The number of mammoth dates sharply jumps up after 15 ka BP, and stays at high levels (av. 7.4 in 1000 years) until 10 ka BP. [...]

    [...]Both of the peaks in the number of mammoth dates in the Laptev Sea collection correspond to periods with a relatively high proportion of xerophilic insects and the presence of steppe species (Fig. 6). A high diversity of insect fauna implies a richer vegetation mosaic, including more extensive areas with better heat supply, occupied by presumably more productive grassland. The number of mammoth dates starts to decrease at the same time as the proportions of those insect groups, reaching its lowest point soon after the period of total absence of steppe species. This pattern allows us to infer that all varieties of tundra–steppe insect assemblages indicate environments tolerable to the woolly mammoth, but that the less diverse insect faunas of the LGM, lacking the most thermophilic beetle species, may suggest less productive, probably just sparser, vegetation, and large grazers seem to have responded by reducing their numbers.

    Timing of events

    A common point made in many papers is that there are significant differences across different regions. Ice sheets don't all move at the same time. Some of what has been presented as "conflict" in other threads (like the Melt Water Pulse thread) are not conflicts at all; merely different times for different events in different regions.

    There's no conflict at all with warming starting at one time and accelerating briefly at another. Here's a very recent paper which gives considerable detail on timing of events and the possible roles of different sheets, and considering the regional variability involved.
    Clark is a major advocate of a Southern source for the MWP-1A. But many other papers argue strongly for a Northern source. Arguments are reviewed by an advocate of the Northern source in
    • Peltier, W.R. (2005) http://dx.doi.org/10.1016/j.quascirev.2004.06.023 [Broken], in Quaternary Science Reviews 24 pp 1655–1671, doi:10.1016/j.quascirev.2004.06.023

    Earth is large; and there are regional differences. Sorting that out is a rich source of open research questions, but its not a sign of some fundamental discrepancy not being acknowledged! There's no reason to expect all changes to be uniform over the globe, and there's no particular special problem with having the onset of retreat for different sheets being at different times, or with the existence of strongly non-linear changes (pulses) at different times.

    Cheers -- sylas
     
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  19. Sep 8, 2009 #18

    Xnn

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    Thanks for the summary sylas;

    Looking over the table of excess sea level, it is surprising to see how large of a contribution Antactica had made. That island is already so loaded with ice that it is hard to comprehend how it could have held even more during the LGM. I think the implication is that there was about 30% more ice in Antarctica than there is now.

    It's also interesting that "All Others" has a greater amount than Greenland.
     
  20. Sep 8, 2009 #19

    sylas

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    Oh yes, definitely. Ice there has gone up and down a lot; especially in the West Antarctic sheet. (WAIS)

    As you have noted yourself, there's a lot more involved in the advance and retreat of ice sheets than simply temperature. The WAIS has been in retreat for a long time, and so it is an open question how much this is impacted by global changes over the last century.

    Yes; in the LGM there were a lot of other smaller ice sheets and glaciers around; and Greenland stands out because it still has a lot of ice retained from the LGM.

    One of the issues for me in putting together that last post was maintaining focus and limiting the amount of information I tried to summarize. Finding that special issue was a help, because it gives lots of review articles and a range of perspectives. The Quarternary Science Reviews gives easy access to tables of contents, and even those who don't have access to the journal can look through the contents of Volume 21, Issues 1-3, Pages 1-454 (January 2002) and pick out promising titles; then hunt around for preprints that are sometimes available. If someone without access has a particular article they'd like summarized, I'm happy to give it a quick look over and my impression of where it stands.

    Cheers -- sylas
     
  21. Sep 9, 2009 #20
    Sorry for the delay, I have too much distraction now to duck into this deeply but I got Clark et al 2009 now. So I'll return to that in detail later.

    Meanwhile I still see a lot of questions. (remember, questions, not claims)

    How could Arctic Siberian summers appear to be warmer than today in the period before the Last glacial maximum around 24 ka Cal BP, the time of the Jarkov and Fishhook mammoth (Lark needles 200k north of the present tree line), Mol et al 2006, Hubberten et al 2004, while the ice sheets in North America were approaching their maximum extent? This is especially awkward, considering the summer insolation minimum for the northern hemisphere at exactly that time (fig 4 Clark et al 2009*):

    2s9ynoi.jpg

    caption:
    ------

    Next, the present antarctic ice sheet contains ice of some one million year old (results EPICA Dome C), the Greenland Ice sheet around 200,000 year) representing about 3000 meters of ice thickness. The Holocene boundary in the -fast accumulating- Greenland ice sheet is at about 1600-1700 meters below the surface of the ice sheet, which is good for about 12,000 years ice accumulation. However in Antarctica (Epica Dome-C) that boundary is only at around 360 meters. So what do we estimate the ice thickness of the advancing ice sheets towards their last glacial maximum positions between 26.5 ka to 19 to 20 ka (Clark et al 2009)? Considering that there are paleobiologic remains below the glacial deposits of about 26-27ka (for instance Matthews et al 1990 ) there wasn't much time to accumulate 3000 meters of ice, which would have been required to find on North America comparable ice volumes with Antarctica, or would it?

    Less relevant to the current topic but essential for understanding why we don't understand what happened, are deep ocean interactions, the 100 ka cycle, not being Milankovitch related and the http://www.moraymo.us/current_projects.php [Broken]? I'll open a few threads on that in due time.

    Then there is also the problem of the Greenland isotopes versus atmospheric methane concentration not concurrent with the global temperature changes as indicated by the ice melting/sealevel rise. Furthermore, it still does not explain why the benthic oceanic isotopes appear to react instanteaneously on melt water at the ocean surface, while the oceanic inertia should have caused not only a significant delay but also a signal weakening (more gradual), which is not visible at all, (fig 4, Clark et al 2009*).

    w71tmt.jpg

    caption:

    *)Both figs and captions: Clark et al 2009 Science 7 August 2009 Vol 325 pp 710-714
     
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