Global Warming, the Blog Epic ~ 06 ~ A Glacial Cycle

This is the sixth in a series of reposts from gregladen.com on global warming.

In the last post in this series I talked about two aspects of large scale climate change: Milankovitch orbital geometry and the cycles of glaciation this effect causes, and the role of plate tectonics and related changes in altering sea and air currents, which in turn determine a great deal about climate change as well.

Now I want to have a quick look at a single glacial cycle (the most recent one of many), and one way in which the cycle is observed in the ancient record, identified, measured, and described.

As discussed earlier, we know that glaciations (glacial cycles, or "ice ages") involve the formation of large continental glaciers, which are in turn made of accumulated precipitation (snow), most of which ultimately comes from the oceans via evaporation. So as water is transferred from the oceans to the land-based glaciers, the glaciers build up and sea level goes down.

Since water can be made of either lighter or heavier isotopes of oxygen, and the lighter-isotope water evaporates more easily, the glaciers are isotropically light. This means, in turn, that the oceans are isotropically heavy. This isotopic bias is preserved in the hard parts of marine organisms that use oxygen from sea water as part of their growth process.

So, how do we know the details of a glacial cycle? Well, first you get a big ocean going boat equipped with drilling/coring machinery, the ability to store cores that are raised from the depth of the ocean, and a bunch of scientific equipment and on-board laboratories. Presumably your boat will also have sleeping quarters, a stock of food, a bathroom, first aid kit, some beer, stuff like that. But the main thing is the big drilling rig, the labs, and the drilling crew, and some scientists. This photograph is of the Joides Resolution, a ship with a long history of service on the high seas. (Image by USGS.)

The drilling process involves driving long tube into the muck on the bottom of the ocean, in the location you've chosen based on the research questions and the likelihood of preservation of sediments. This is called "coring" and the muck-tube you get is called a "core." A very large number of deep ocean cores have been lifted from all of the world's oceans. Special emphapsis has been placed in certain areas, such as the North Atlantic or the Indian Ocean, because of the central role that currents in these areas play in climatic reconstruction. If you looked at a map of the world with the ocean cores indicated on it, you's be pretty impressed with the number. This cut-out from Google Map shows the exact location of North Atlantic Core RC01-1120, which I believe was lifted in the early 1970s.

So, you pull out the sample and analyze it using all of the available techniques. Typically, the core is split in half and one half is stored in a giant refrigerator to serve for all time as "witness" to the other work you are doing on the core, possibly to be analyzed in a limited way in the future. Many different techniques are used to analyze the core, including some that are not destructive, and others that are. The trick to doing this is to organize access to the core among different scientists so that one person's work does not mess up the work of another. This is a core shown is five segments with a few key landmarks indicated. You can see that each core segment is about 150 cm long.

One technique that is applied is to measure, using a mass spectrometer, the ratio of heavy (18) vs light (16) oxygen in the hard body parts of the organisms you find in the core. You do this over very short intervals. For instance, the following lists the O 18/16 values (Delta-18 values, as they are called) of these organisms from core RC01-1120 (from the North Atlantic).

Depth (cm);Delta 18 value - - 5;1.87 - 10;1.96 - 15;1.97 - 20;1.96 - 25;2.16 - 30;2.38 - 35;2.32 - 40;2.6 - 45;2.74 - 50;2.98 - 55;2.95 - 60;3.37 - 65;3.28 - 70;3.53 - 75;3.34 - 80;3.36 - 85;3.43 - 90;3.34 - 95;3.31 - 100;3.29 - 105;3.32 - 110;3.22 - 115;3.07 - 120;3.04 - 125;3.07 - 130;3.19 - 135;2.96 - 140;3.11 - 145;2.98 - 150;3.05 - 155;3.01 - 160;2.94 - 165;2.98 - 170;2.95 - 175;2.89 - 180;2.9 - 185;2.66 - 190;2.86 - 195;2.87 - 200;2.82 - 205;2.82 - 210;3.03 - 215;2.76 - 220;3.08 - 225;3.15 - 230;2.84 - 235;2.98 - 240;2.85 - 245;2.82 - 250;2.71 - 255;2.59 - 260;2.54 - 265;2.55 - 270;2.59 - 275;2.44 - 280;2.43 - 285;2.42 - 290;2.37 - 295;2.4 - 300;2.71 - 305;2.61 - 310;2.68 - 315;2.58 - 320;2.68 - 325;2.54 - 330;2.6 - 335;2.4 - 340;2.54 - 345;2.48 - 350;2.42 - 355;2.4 - 360;2.48 - 365;2.45 - 370;2.48 - 375;2.46 - 380;2.62 - 385;2.5 - 390;2.61 - 395;2.42 - 400;2.33 - 405;2.14 - 410;2.23 - 415;1.83 - 420;1.89 - 425;1.79 - 430;1.91 - 435;2.35 - 440;2.72 - 445;2.82 - 450;3.18 - 455;3.2 - 460;3.44 - 465;3.31 - 470;3.54 - 475;3.18 - 480;3.37 - 485;3.42 - 490;3.1 - 495;3.14 - 500;3.38 - 505;3.25 - 510;3.19 - 515;3.27 - 520;3.21 - 525;3.22 - 530;3.24 - 535;3.27 - 540;3.21 - 545;3.15 - 550;3.32 - 555;3.15 - 560;3.21 - 565;3.13 - 570;3.06 - 575;2.9 - 580;2.91 - 585;2.87 - 590;2.81 - 595;2.81 - 600;3.02 - 605;2.85 - 615;2.61 - 620;2.59 - 625;2.55 - 630;2.43 - 635;2.32 - 640;2.18 - 645;2.17 - 650;2.14 - 655;2.41 - 660;2.52 - 665;2.49 - 670;2.45 - 675;2.13 - 680;2.31 - 685;2.24 - 690;2.2 - 695;2.47 - 700;2.7 - 705;2.55 - 710;2.62 - 715;2.62 - 720;2.86 - 725;2.85 - 730;2.72 - 735;2.74 - 740;2.78 - 745;2.46 - 750;2.46 - 755;2.45 - 760;2.57 - 765;2.38 - 770;2.39 - 775;2.62 - 780;2.71 - 785;2.67 - 790;2.86 - 795;3.03 - 800;2.92 - 805;2.99 - 810;2.94 - 815;3.05 - 820;2.97 - 825;2.88 - 830;2.84 - 835;2.91 - 840;3.04 - 845;2.94 - 850;2.85 - 855;2.95 - 865;2.43 - 895;2.91 - 900;2.79 - 905;2.8 - 910;2.58 - 915;2.56 - 920;2.57 - 925;2.43 - 930;2.43 - 935;2.5 - 940;2.67 - 945;2.67 - 950;2.69

So that's about 30 feet of muck divided into 183 segments each with a Delta-18 value.

Here I graph the topmost 84 values from these cores. These values are estimated to represent about the last 120 years of time. On this graph, higher values correspond with warmer periods ... periods when there is less of the world's water trapped in glacial ice ... and lower values represent cooler periods ... when less of the world's water is in the glaciers.

This is not a fancy reconstruction or a curve based on lots of different data nice and normalized and fixed up. This is the simple raw data from one single core. If you look at a lot of different cores, you see basically the same thing again and again and again, regardless of whether the cores are from the North Atlantic, the Pacific, the Indian Ocean, anywhere else in the ocean, or even in a lake (except the lake may be measuring rainwater isotopes, so the calculation is somewhat different.) This is important, because it confirms that this is not an artifact of some local effect. Or some crazy rabbit-out-of-the-hat idea cooked up by wacky creationists such as "all the muck on the bottom of the ocean ended up there as a result of the Noachean Flood, and the oxygen isotope ratios are because of ... ah .... I'll get back to you on that..."

Notice that the most severe part of the glacial period comes just before a very rapid warming to present conditions. Notice that present roughly conditions occurred last about 120 thousand years ago. Notice that most of the time shown on this graph it is not warm like it is now. Notice the last bump at the very end, on the left side of the graph ... that's probably a global warming signal.

Also, since this is essentially a graph of how much water is in glacial ice, it is also a graph of sea level! I've adjusted the y-axis so that "up" is warm. This also means that "up" is high sea levels. So, for much of this time, sea levels were much lower than they are now, meaning that much more land was exposed.

Cape Cod Bay, New York Harbor, San Francisco Bay, Puget Sound ... to name a few places, were high and dry during lowest sea levels. Jamaica and Cuba were high points surrounded by dry land straddling a large broad valley containing a dimunative arm of the Caribbean, and the Gulf of Mexico was way small.

SECMAP data source: "Oceanic Response to Orbital Forcing in the Late Quaternary:
Observational and Experimental Strategies", by J. Imbrie, A.
McIntyre, and A. C. Mix. in 'Climate and Geosciences, A Challenge
for Science and Society in the 21st Century', A. Berger, S. H.
Schneider, and J.-C. Duplessy, eds., D. Reidel Publishing Company.