Abstract. The aim of this project was to establish past variations in the main
oceanographic and climatic features of a transitional semiarid ecosystem on
the north-central Chilean coast. We analyzed recent sedimentary records
retrieved from two bays, Guanaqueros and Tongoy (30∘ S), for
geochemical and biological analyses, including the following: sensitive
redox trace elements, biogenic opal, total organic carbon (TOC), diatoms
and stable isotopes of organic carbon and nitrogen. Three remarkable periods
were established with different environmental conditions and productivities:
(1) > 6600 cal BP, (2) 4500–1800 cal BP and (3) 140 cal BP to
the present (2015 CE). The first period was characterized by a remarkably
higher productivity (higher diatom abundances and opal) in which large
fluxes of organic compounds were also inferred from the accumulation of
elements, such as Ba, Ca, Ni, Cd and P, in the sediments. Meanwhile,
significantly reduced conditions at the bottom of the bays were suggested
based on the large accumulation of Mo, Re and U, showing a peak at 6600 cal BP, when sulfidic conditions could have been present. According to the
pollen moisture index (PMI), this was also identified as the driest interval.
These conditions should be associated with an intensification of the
Southern Pacific Subtropical Anticyclone (SPSA) and stronger southerly western
winds, emulating La Niña-like conditions, as has been described for
the SE Pacific during the early Holocene and part of the mid-Holocene.
During most of the second period, lower productivity was observed; however,
a small increase was identified between 3400 and 4000 cal BP, although lower
amounts of diatom (valves g−1) and nutrient-type metal accumulations
were evident. Anoxic conditions at the bottom of the bays changed to an
almost stable suboxic condition during this time interval. The third period
was marked by intense oxygenation after 1800 cal BP, as observed by a
drastic change in the accumulation of U, Mo and Re. This was followed by a
return to more reduced conditions over the past 2 centuries, characterized
by a small productivity rise after ∼140 cal BP, as suggested
by the opal accumulations. Overall, lower primary productivity, lower
reduced conditions at the bottom and higher-humidity conditions were
established after 6600 cal BP to the present. We suggest that the
oxygenation might be associated with a weak effect from the oxygen minimum
zone over the shelf and intensified El Niño activity, introducing
oxygenated waters to the coastal zones through the propagation of equatorial
waves and establishment of conditions that reduced the primary productivity
from the mid-Holocene toward the beginning of the modern era.
Growth and Spawning Dynamics of Southern Flounder in the North-Central Gulf of Mexico
