Natural Science Forum / Biology / Paleontology / May 2006

 D "nickname" D:
 Hmm. I see that I'd better reply to your last posting in detail.
 FWIW, the Tethys Sea ceased to exist long before humans
began to exist, in fact before the age of earliest ape fossils
around the Mediterranean Sea, IIRC, certainly more than
5 million years ago .

<BEGIN QUOTE>
Daryl, in a nutshell, would this be right?
About 8ka, a big glacial lake in Canada broke thru a dam and flushed
the gulf stream chilling west coast of Europe, approximately the same
time the Caspian was flooding into the Black due to glacial melt,
causing the Black to overflow into the Mediterranean... however both
the Black and Medit. were about the same surface level, so due to
tectonic and hydrological actions (Canadian lake flush) occasionally
Medit. waters moved back into the Black.
If the above is true, then my question is, previous to the glacial
melt, was the Black a saline sea or freshwater or like the Caspian (1/3

salinity of Ocean).   DD
<END QUOTE>

 Hokay, more or less, but not quite exactly right.
 Here's a simplistic version of the Black Sea's recent history:
 At Last Glacial Maximum (say, at about 18 ka),
the Black Sea was probably overflowing into the Mediterranean Sea,
but not _necessarily_ through the Bosphorus Strait. The most likely
candidate for an alternative connection is via Izmit Bay and the
Sakarya River valley. This overflow may have carried some smallish
component of glacial meltwater from the Fennoscandian Ice Sheet
(FIS) via the Dneiper river. The level of the Mediterranean Sea (MS)
would have been more than 100 m lower than it is today.
 Sometime near the Younger Dryas episode (which was possibly
related to an earlier partial drainage of Glacial lake Agassiz, through

the Mackenzie River valley), the level of the Black Sea (BS) seems
to have started to drop (maybe to -150 m), and thus to have stopped
overflowing into the Mediterranean Sea.
 Then the Caspian Sea (CS) rose enough to overflow through the
Kuma-Manych Channel (named after the rivers at either end)
north of the Caucasus Mountains, without eroding a really obvious
channel (and therefore not with catastrophic vigour). The Aral Sea
also expanded at that time, so the CS overflow was maybe the
result of climatic cooling leading to reduced evaporation and/or
extra precipitation.
 That CS overflow may have raised the level of the BS above
the level of the Bosphorus sill ( -35 m), and there is evidence from
a now-submerged delta at the south end of the Bosphorus that
the BS was overflowing through the Bosphorus by about 10 ka.
At that time, the MS level was slowly rising because of the input
of glacial meltwater into Ocean, but was lower than the Bosphorus
sill.
 Then the CS overflow ceased, and the BS level may have dropped
again, this time to about -85 m.
 The MS level rose to the Bosphorus sill about 9ka, and from then on,
there is evidence of saltwater incursions into the BS.
 Ryan says, in his Letter on the subject of a revised BS Flood
in Earth and Planetary Science Letters
(not necessarily critically reviewed before publication)
that about 8.2 ka (or 8.4ka, or 9ka; the dates he presents are
complicated and non-uniform),
the MS started to overflow into the BS and quickly raise the BS level
from
its later -85 m lowstand to something like the -35 m Bosphorus sill
level.
he might be right about that, except that his data can be interpreted
to
allow up to three centuries for that rise in BS level, and there is no
mention
in his revised BS Flood hypothesis for the catastrophic scouring-out of
the
Bosphorus Strait that was a linchpin of the earlier version put forth
by him
and Pitman (that catastrophic-breaching-of-the-Bosphorus scenario was
in
their book, in media interviews, and on a TV program, but not in any
scientific journal).
 There is evidence from a delta at the mouth of the Sakarya River (at
the
BS shore, east of the north end of the Bosphorus) that BS level was at
or
above Bosphorus sill level from 8.2 ka onward
(and possibly earlier, because that date was from the bottom of the
dated
material, and there is no information there about what happened
earlier),
and slowly rose with MS level, until it reached -18 m at about 7.2 ka
(which is the date of the BS Flood (Mk. I) of Ryan and Pitman, when
they
had BS level at -150 m).
 Global sea level, MS level, and BS level all would have stabilised at

modern sea level about 6000 years ago, when significant input of
glacial
meltwater ended, which was when the last remnants of North America's
Laurentide Ice Sheet (LIS) disappeared (except for two small ice caps
on
Baffin Island, which persist today).

 Ryan indicates in that BS Flood (Mk. II) Letter that at the time of
their
supposed BS Flood (Mk. I), the BS level was the same as MS level, and
that the BS was overflowing into the MS, and that the "sudden"
appearance
of saltwater shells at about 7.2 ka (not really "sudden", tho) that
they noted
was due to "a salinity change", i.e. an alteration of water quality at
depth,
not at the surface, one that did not involve any flooding of the BS
basin at all.
 Pitman has not publicly disagreed with him.
 They are no longer defending their BS Flood (Mk. I) from the
rebuttals of
those who know more about the history of the Black Sea than they
bothered
to find out (one of the supposed co-authors of their sketchy
one-and-only
scientific article on the subject, Naci Gorur, soon was seen to be one
of the
rebutters; the only mention of him in their book is an indication that
they had
lunch at the same place one day).

 The last drainage of Glacial Lake Agassiz may have increased the rate
of
saltwater incursion into the BS, or even started flow of MS water
through the
Bosphorus, but it did not initiate that saltwater incursion into the
BS. It
does roughly coincide with Ryan's BS Flood (Mk. II), but only if you
hold your
tongue just right and squint at it.

 The salinity of the BS varied over time:
at LGM, it was fresher than today, but not as fresh as the rivers that
fed it;
it would have gotten saltier as its water evaporated enough to drop to
a
-150 m lowstand;
addition of CS overflow (itself fresher than today, but not as fresh as
Volga
River water) would have reduced BS salinity again;
BS salinity would have increased again as it dropped to a -85 m
lowstand;
BS salinity would have increased dramatically at its floor, as denser
MS
saltwater sank below the fresher water above;
at some time after 9ka, the salinity of the BS would have stabilised to

what it is today: salty at depth, a brackish (half-salty) mix of
saltwater and
fresh river water in the upper levels, and relatively fresh at river
mouths
(where survivors from fresher-water times, relatives of fresher-water
shellfish
that left fossils in earlier times, can still be found).
 Today, the lower level of the Bosphorus Strait is MS saltwater
flowing
northward, and the upper level is BS brackish water flowing southward.
 The BS outflow is about double the MS inflow; the difference between
the
flow volumes is about the same as the total river inflow into the BS,
so that
the flows through the Bosphorus are balanced, except for the outflow
from
rivers. That two-level flow has been happening continuously for at
least 5000
years, i.e. for at least as long as global sea level has been at a
stable level.

 This is what we know from the fossil record.

 Tectonic movement is a minor factor in the history of the Black Sea,
unless you want to speculate that an earlier Izmit Bay / Sakarya River
valley connection between the BS and the MS was closed by uplift of
the floor of the middle of that channel.
 There is no evidence of a rapid drop in the floor of the Bosphorus
which
would have created a sudden opening of that Strait; rather, it
originated as
a rift valley with rivers flowing away from a divide along the line of
the
mountains which rise above the south shore of the BS. The divide was
a high point in the floor of that rift valley before it subsided enough
for the
northern river valley to extend into the headwaters of the southern
river valley,
enough so that the modern high point in the floor of the rift valley,
the sill near
the Golden Horn promontory of Istanbul, is nearly at the southern end
of the
rift valley.
 At some point, the sill became lower than the level of either the BS
or the
MS, and it then became a conduit for outflow from the BS and/or inflow
from
the MS.
 That sill is covered with sandy sediment that was probably derived
from
local rivers, at times when conduit flow through the rift valley was
slow or
non-existent. There is no evidence that the sill area was scoured by a
sudden catastrophic rush of water from either direction.

 Capice?

-
Daryl Krupa