Agriculture Reference
In-Depth Information
contended that a large area north of the contemporary MTC in southwest
Australia was MTC at 25 Ma or earlier (Hopper & Gioia
2004
). However,
aseasonal tropical climates were the norm over much of southern Australia at this
time (Christophel & Greenwood
1989
). Using sea surface temperatures and Ant-
arctica glaciation, Linder (
2005
) suggested a MTC for the Cape region of South
Africa during the early Oligocene followed by more mesic conditions in the early
Miocene.
In North America, paleosol characteristics indicate the presence of a summer-
drought climate in southern Colorado during the late Oligocene (Wolfe and
Schorn
1989
). This seasonality potentially existed throughout the southwestern
portion of interior North America and these lower latitudes were possibly more
arid, which would be consistent with the very limited Oligocene fossil floras for the
region. The Pacific slope of California was much moister and definitely not MTC
at that time, as revealed by the substantial subtropical macrofossil record
(Axelrod
1973
).
Miocene
The Miocene began with moderate temperatures and increasingly became more
seasonal in both temperature and precipitation. These were global patterns driven
by changes in sea surface temperatures and Antarctica glaciation as well as
orogenic uplift.
Western North America has a rather extensive Tertiary macrofossil record,
making climate reconstruction more feasible than in some other regions. Miocene
temperature and rainfall heterogeneity was enhanced by the Sierra Nevada rain
shadow that had developed by the mid-Miocene (Crowley
et al.
2008
). However,
such topographic influences appear to have been present much longer, as recent
studies point to an earlier Late Cretaceous uplift for the northern Sierra Nevada
(Busby & Putirka
2009
; Cassel
et al.
2009
), where the present-day range repre-
sented the western edge of the Nevadaplano that extended east across the present
Great Basin (Henry
2009
). Today the northern range reflects a reduction in mean
elevation from early Tertiary levels but greater relief (Hren
et al.
2010
).
Using an abundant macrofossil record, Axelrod (
1989
) concluded that the
MTC in California was a Quaternary phenomenon and that Tertiary climates
were dominated by summer rains. He took great stock in the observation that
mid and late Miocene floras had taxa that were allied to contemporary species
now confined to summer-rain climates (e.g. Eastern Deciduous Forest taxa:
Carya
,
Diospyros
,
Nyssa
,
Robinia
,
Ulmus
, and
Zelkova
, and subtropical taxa:
Persea
,
Magnolia and Cinnamomum
, Raven & Axelrod
1978
). Even for fossil
floras with many contemporary chaparral and sclerophyllous woodland taxa,
he considered any representation of summer-rain-allied taxa as indicative of a
summer-rain climate.
One example is the 7-Ma Piru Gorge flora in which 5 of the 22 species
were considered summer-rain indicator species; based on this, Axelrod (
1982
)
