Time-based correlation of biogenic, lithogenic and ... - Springer Link

Estuaries Vol. 17, No. 4, p. 873-885

December 1994

Time-Based Correlation of Biogenic, Lithogenic and Authigenic Sediment Components with Anthropogenic Inputs in the Gulf of Mexico NECOP Study Area TERRYA. NELSEN1

NIDIA R O M E R

National Oceanic and Atmospheric Administration Atlantic Oceanographic and Meteorological Laboratory Ocean Chemistry Division 4301 Rickenbacker Causeway Miami, Florida 33149

CARLOS ALVAREZ-ZA~mAN

Department of Marine Geology and Geophysics Rosenstiel School of Marine and Atmospheric Sciences University of Miami 4600 Rickenbacker Causeway Miami, Florida 33149

PAT BLACKWELDER

TERra HOOD

SIMONE METZ

Department of Marine Geology and Geophysics Rosenstiel School of Marine and Atmospheric Sciences University of Miami 4600 Rickenbacker Causeway Miami, Florida 33149

Department of Oceanography Florida Institute of Technology Melbourne, Florida 32901

BR~NT MCKEE

Louisiana Universities Marine Consortium 8124 Highway 56 Chauvin, Louisiana 70344 Hypotheses related to variability in seasonal hypoxic conditions, coastal nutrient e n h a n c e m e n t , and offshelf transport o f carbon on the Louisiana continental shelf were tested by characterization o f biogenic, lithogenic, and authigenic c o m p o n e n t s f r o m two shelf and one Mississippi Canyon sediment cores. T h e authigenic-phase glauconite occurs above detection limits only in the core f r o m the hypoxic area. A major increase in glauconite concentration was coincident with the onset (--1940) o f the increased use o f commercial fer@lizers in the U n i t e d States. In the same hypoxic-area core, benthic foraminifera species diversity decreases upcore from approximately the turn o f the century to the p r e s e n t in a m a n n e r concurrent with glauconite and fertilizer increases. A subset o f opportunistic benthic foraminifera species, known to b e c o m e m o r e p r o m i n e n t in stressed environments (i.e., hypoxic), increased upcore f r o m - 5 2 % o f the total population at core b o t t o m to - 9 0 % at core top. T h e s e benfltic foraminifera population and diversity changes were not apparent in a "control" core outside the area o f d o c u m e n t e d hypoxia. Seaward o f the shelf, in the Mississippi Canyon, coincident increases in s e d i m e n t accumulation rate, percentages o f coarse fraction and o f organic carbon at core top indicate increased offshelf transport o f carbon and o t h e r components. Quartz percentages indicate that episodic down-canyon transport has b e e n active to core b o t t o m (prior to the m i d 1800s).

ABSTRACT:

Introduction

the r u n o f f of - 4 1 % of the continental United States (van der Leeden 1975; SAUS 1991). This r u n o f f constitutes the largest single source of fresh water and sediment to the United States coastal zone (Meade and Parker 1984). Since the onset o f the Industrial Revolution and agricultural develo p m e n t within .America's heartland, the Mississippi River has become a conduit for anthropogenic by-

T he Mississippi River has a drainage basin that spans westward from eastern New York to Wyoming and Montana, and from southern Canada to the Guff o f Mexico, t hr oughout which it integrates t C o r r e s p o n d i n g author. 9 1994 Estuadne Research Federation

873

0160-8347/94/040873-13501.50/0

874

T.A. Nelsen et al.

products. T h e s e are i n t r o d u c e d into the Gulf o f Mexico at two p o i n t sources: the main stem o f the Mississippi River at the b i r d f o o t delta, a n d the Atchafalaya River. I n c r e a s e s in a n t h r o p o g e n i c a l l y - d e r i v e d riverb o r n e nutrients (e.g., nitrates) over the last several decades (Bratkovich a n d Dinnel 1992) have resulted in changes in river water quality a n d hypothesized modifications in coastal t b o d webs ( T u r n e r a n d Rabalais 1991). Since 1985, o n e such coastal water-quality p a r a m e t e r , s e a s o n a l b o t t o m - w a t e r hypoxia, has b e e n m o n i t o r e d a n d d o c u m e n t e d over extensive areas of the Louisiana shelf (Rabalais et al. 1991, 1992). An a p p a r e n t linkage between these observations has led to the N u t r i e n t E n h a n c e d Coastal O c e a n Productivity (NECOP) P r o g r a m ' s central hypothesis that " a n t h r o p o g e n i c n u t r i e n t inputs have e n h a n c e d coastal o c e a n productMty with s u b s e q u e n t i m p a c t on coastal ocean water quality a n d living resource yields." However, d o c u m e n t a t i o n of such linkage is a challenge. Unlike Mississippi River s e d i m e n t a n d water discharge records, which go back to the early 1930s a n d late 1800s respectively, decadal to c e n t u r y scale prim a r y productivity a n d water quality m e a s u r e m e n t s f r o m the Louisiana coastal zone do n o t exist. Therefore, direct long-time-scale evaluation of shelf ecosystem variability a n d o f changes in the Mississippi River's n u t r i e n t Ioadings c a n n o t be m a d e f r o m existing datasets. Sediments on the Louisiana shelf adjacent to the m o u t h o f the Mississippi River potentially contain a c o n t i n u o u s r e c o r d of such changes a n d may be evaluated in a m a n n e r a n a l o g o u s to a long-term m o n i t o r i n g study. T h e validity of such an a p p r o a c h has b e e n d e m o n s t r a t e d for a n t h r o p o g e n i c Pb in the sediments adjacent to the Mississippi Delta (Trefry et al. 1985). Sediments in that study rec o r d e d m o r e t h a n a c e n t u r y o f steady increase in Pb f r o m the onset o f the Industrial Revolution. Coincident with regulation o f leaded gasoline in the m i d 1970s, a n t h r o p o g e n i c Pb steadily decreased. T h e Retrospective Analysis g r o u p o f the N E C O P P r o g r a m has u n d e r t a k e n a chronological study o f Louisiana c o n t i n e n t a l shelf sediments. In a complex sedimentological setting such as a delta envir o n m e n t , no a t t e m p t was m a d e to characterize the entire e n v i r o n m e n t b u t r a t h e r to focus on three areas: 1) an area of known seasonal hypoxia, 2) an area n o t i m p a c t e d by d o c u m e n t e d seasonal hypoxia, and 3) an a r e a o f potential offshore t r a n s p o r t o f carbon. T h e objectives were to d e m o n s t r a t e the p r e s e n c e of diagnostic inorganic a n d / o r organic m a r k e r s within the s e d i m e n t r e c o r d that m a y provide a basis f o r d i s c r i m i n a t i o n o f l o n g - t e r m changes in river water quality as well as Louisiana shelf productivity a n d hypoxia. Specifically, o u r

hypoxia-related efforts were directed at e x a m i n i n g the tollowing hypotheses:

Hypothesis i. Hypoxia Indicators. Byproducts of hypoxia-anoxia events have left characteristic m a r k e r s that record a time history in the s e d i m e n t record. IIypothesis 2. Community Structure. O b s e r v e d riverine n u t r i e n t e n h a n c e m e n t has p r o m o t e d a shift in benthic f o r a m i n i f e r a c o m m u n i t y structure on the Mississippi-Louisiana continental shelf a n d this shift is p r e s e r v e d in the s e d i m e n t record. A third hypothesis, dealing with carbon accum u l a t i o n resulting f r o m n u t r i e n t e n h a n c e m e n t , a n d its diagenesis, is addressed in a c o m p a n i o n pap e r (Eadie et al. 1994). However, a n o t h e r aspect, the offshore t r a n s p o r t of e n h a n c e d coastal c a r b o n p r o d u c t i o n to the continental slope, is c o n s i d e r e d here. This process is i m p o r t a n t for its implications to the long-term storage a n d removal of c a r b o n f r o m the global c a r b o n cycle. C a r b o n is transported to d e e p water d e p o c e n t e r s tor storage a n d removal f r o m recycling within the shelf ecosystem. Prior investigation o f offshelf t r a n s p o r t in the Atlantic coastal region (Walsh et al. 1981) hypothesized that half or m o r e o f the shelf productivity is carried past the shelf break. S u b s e q u e n t studies have q u e s t i o n e d the m a g n i t u d e of this hypothesized process (Rowe et al. 1986), a n d the issue still remains unresolved. O u r results to date are based on three cores recovered f r o m the I,ouisiana coastal zone. T h e objective of this p a p e r is to p r e s e n t new data conc e r n i n g h y p o x i a i n d i c a t o r s , shifts in b e n t h i c f o r a m i n i f e r a c o m m u n i t y structure, a n d offshore s e d i m e n t a n d c a r b o n transport.

Methods To test o u r hypotheses a multidisciplinary app r o a c h was utilized. Stratigraphy, radioisotopebased geochronology, organic and inorganic chemical analyses, detailed characterization o f the s e d i m e n t ' s coarse-grained fraction, and foraminifera biostratigraphy were e x a m i n e d . Core site selection was based on two criteria: 1) areas known for the p r e s e n c e a n d absence of docu m e n t e d seasonal hypoxia and, in addition, an area o f anticipated offshore s e d i m e n t a n d c a r b o n transport; 2) a region in which s e d i m e n t accumulation rates are a d e q u a t e to allow dccadal to century time-scale resolution. Since 1985 shelfwide m o n i t o r i n g of water-colu m n oxygen levels has allowed m a p p i n g of seasonal hypoxic conditions on the Texas-Louisiana continental shelf (Rabalais et al. 1991). To fulfill the first criterion, we analyzed the yearly distribution of hypoxia locations a n d identified coring sites that persistently e x p e r i e n c e seasonal hypoxia. An area

Sediment Trends and Anthropogenic Influences

Ig"

Fig. 1. Index map of the study area with core site locations. was also chosen at which no seasonal hypoxia has ever b e e n d o c u m e n t e d . T h e coring site for evaluating the time history of offshore s e d i m e n t a n d c a r b o n t r a n s p o r t was selected based on two criteria: 1) previously m o n i t o r e d (Nelsen a n d Trefry 1986) offshelf t r a n s p o r t of s u s p e n d e d particulate m a t t e r a n d 2) high levels (-----20%) of fresh terrestrial c a r b o n (based on C-isotope analysis) in the benthic b o u n d a r y layer (B. Eadie personal comm u n i c a t i o n ) . Previous work (Trefry et al. 1985; Nelsen a n d Trefry 1986; B. McKee u n p u b l i s h e d data) has clearly d e m o n s t r a t e d that shelfwide interpretable g e o c h r o n o l o g i e s can be o b t a i n e d f r o m the p r o p o s e d study area with consistent a n d interpretable s e d i m e n t a c c u m u l a t i o n rates a n d crossshelf trends (Nelsen a n d Trefry 1986; B. McKee u n p u b l i s h e d data). T h r e e cores fulfilled the above criteria (Fig. 1). T h e core 10 site was chosen f r o m an area of persistent seasonal bottom-water hypoxic conditions. Core 7, r e c o v e r e d outside o f the hypoxic area, was collected d u r i n g a p r i o r p r o g r a m . It served n o t only as this project's initial " p r o o f of c o n c e p t " core but also as a " c o n t r o l " core for c o m p a r i s o n with hypoxic-area core 10. To e x a m i n e offshore c a r b o n transport, core 8 was r e c o v e r e d seaward o f the continental shelf in the thalweg of the Mississippi Canyon. Sediments were r e c o v e r e d with a 25 cm • 25 cm • 60 cm c u s t o m - m a d e , stainless steel b o x core. U p o n retrieval, water t r a p p e d at the box-core top was carefully s i p h o n e d o f f a n d the s e d i m e n t top was inspected for evidence o f disturbance. In m a n y cases, traces o f b e n t h i c activity, such as w o r m trails, were observed, suggesting that the true sedimentwater interface was recovered. Only cores that m e t this standard were subsampled. S u b s a m p l i n g was

875

d o n e with two to f o u r . j u x t a p o s e d 3-inch d i a m e t e r plastic p u s h c o r e barrels, where subcore centers were s e p a r a t e d by -63 Ixm for coarse-grain analyses. Relative a m o u n t s of coarse, fine particles were determ i n e d a n d the coarse (->63 p,m) fraction retained for f u r t h e r characterization. T h e fine fraction was dried a n d saved for future analysis. T h e ->63 p.m

876

T.A. Nelsen et al.

TABI.E 1. G e n e r a l core i n f o r m a t i o n . Water

Core ID Core 10 Core 7 Core 8

L a t i t u d e Longitude 29 05.22 28 37.90 28 14.60

TABLE 2. Average ~ core s e d i m e n t parameters. (2)re

Depth Length (m) (cm) Kin"

89 43.48 89 43.00 89 32.57

29 104 933

50 30 50

33 42 76

Comment Hypoxic a re a O u t e r shelf Mississippi Canyon

:' Distance from closest distributary m o u t h .

Cores & Intervals

Accuinll- Coarse lation SediRates ment (cm yr -I) %

Quartz

Glaucconite

3.7

19615

2534

0.8

201

PlankOrganic tic Benthic C a r b o n F o r a m s Forams %

H y p o x i a Core 10 0 - 5 0 cm

0.55

4

1771

b

75

214

1.10

863 601 971 948

195 319 224 142

1.12 1.34 1.25 1.01

O u t e r Shelf Core 7 0-30 c m

0.37

c

Mississippi C a nyon Core 8

fraction was characterized for b o t h lithogenic a n d biogenic particles. For each l-cm interval, 3 0 0 1,000 grains p e r sample aliquots of the > 6 3 ~ m fraction were categorized (quartz, glauconite) using a light microscope. T h e s e data were n o r m a l ized to grains p e r g r a m o f d r y sediment. Glauconite grains were first isolated by light m i c r o s c o p y a n d subsequently analyzed with an ISI m o d e l DS-130 s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) e q u i p p e d with a T r a c o r N o r t h e r n m o d e l TN-2000 energy dispersive analysis (EDA) system. In addition, a p p r o x i m a t e l y 300 total benthic a n d planktic f o r a m i n i f e r a were picked f r o m selected samples a n d identified to the species level. To evaluate shifts in b e n t h i c f o r a m i n i f e r a biodiversity, t h e S h a n n o n - W i e n e r I n f o r m a t i o n Function was utilized (Patrick 1983). This index i n c o r p o r a t e s a m e a s u r e o f species evenness as well as n u m b e r of species, such that c o m m u n i t i e s with m a n y species o f equal-size p o p u l a t i o n s have the highest index. T h e index o f diversity (H) is c o m p u t e d f r o m the following expression: N

H = -s

piln Pi i=l

where N is the n u m b e r of species, a n d p~ is the p r o p o r t i o n o f the total n u m b e r o f individual species that b e l o n g to the ith species (MacArthur 1983). In this text, H will be r e f e r r e d to as the S h a n n o n - W i e n e r Diversity I n d e x (SWDI). T h e organic c a r b o n (OC) c o n t e n t o f s e d i m e n t s a m p l e s was d e t e r m i n e d u s i n g a C a r l o - E r b a NA1500 nitrogen-carbon-sulfur analyzer following manufacturer's instructions. Sediment aliquots were p r e t r e a t e d with H 2 S O 4 a n d oven-dried to remove CaCO3. A p p r o x i m a t e l y 10-15 m g of acidtreated s e d i m e n t were weighed into tin cups a n d c o m b u s t e d at 1,000~ T h e precision ( < 2 % ) was m o n i t o r e d by analyzing n u m e r o u s samples in triplicate. T h e accuracy o f the O C analyses was determ i n e d using standard BCSS-1 s e d i m e n t issued by the National Research Council o f C a n a d a (NRC). Values o b t a i n e d for this standard s e d i m e n t agree within the m e a n a n d standard deviations o f NRC p u b l i s h e d values.

All Data 0-9 c m 9-14 cm 14-40 cm

0.199 0.047 --

0.9 1.6 0.9 0.7

29 62 12 18

--~ c c c

% or g r a i n s / g r a m of s e d i m e n t . b No data.

c Below detection (63 [zm sedi m e n t fraction, as well as thc O C % for each core. Profiles for excess 21~ in cores 10, 7, a n d 8 are shown in Figs. 2A-C respectively. Core 10, ~ 3 3 km f r o m Southwest Pass, a n d in an area o f persistent seasonal hypoxic conditions, displayed traces of b i o t u r b a t i o n in X-radiographs. Mixing was minimal, with the sediments retaining clearly discernible zones o f diffuse laminations t h r o u g h o u t the core indicating that bioturbation, if any, was minimal. This was c o n f i r m e d by the quality o f the excess 21~ data (Fig. 2A). A s e d i m e n t a c c u m u l a t i o n rate o f 0.55 cm yr -1 was calculated for core 10 based o n a sampling interval o f 1 cm, in the top 10 cm, a n d 2-cm intervals t h e r e a f t e r (n = 26, Fig. 2A). T h e age resolution is - - 2 - 5 yr and, based on the a c c u m u l a t i o n rate o f 0.55 cm yr 1, this equates to --1900 at core b o t t o m (50 cm). Thus, core 10 records an ideal time span to test o u r hypotheses. Core 7, n e a r the shelf edge at - 4 2 k m f r o m Southwest Pass, was devoid of either p r i m a r y or s e c o n d a r y s e d i m e n t a r y structures in the top 10 cm. Below this, a n d to core b o t t o m ( - 3 0 cm), the X-rad i o g r a p h s displayed a m o t t l e d a p p e a r a n c e . Sedim e n t a c c u m u l a t i o n core 7 was 0.37 cm yr -l based on 2-cm sampling intervals (n = 9, Fig. 2B). An estimated c o r e - b o t t o m date o f - 1 9 0 0 allows for direct t e m p o r a l c o m p a r i s o n between this core, outside o f d o c u m e n t e d hypoxic conditions, a n d core 10 within the seasonal hypoxic area.

Sediment Trends and Anthropogenic Influences

87I

Fig. 2. Excess 21~ curves for (A) core 10 from an area of chronic se&sonal hypoxia, (B) core 7 on the outer shelf, and (C) core 8 from the thalweg of the Mississippi Canyon.

A t - 7 6 k m f r o m S o u t h w e s t Pass, c o r e 8 (Mississippi Canyon) revealed a three-layered sedimentary sequence based on X-radiograph densities. W i t h i n t h e 0 - 9 c m z o n e , r e m n a n t s o f l a y e r e d zon a t i o n w e r e r e t a i n e d in a d d i t i o n to s u g g e s t i o n s o f limited bioturbation. Between -9 cm and -14 cm, a s u b t l e d i f f e r e n c e in X - r a d i o g r a p h density, a n d a d i s t i n c t l a c k o f b i o t u r b a t i o n a c c o m p a n i e d by t r a c e s o f f i n e l a m i n a t i o n s , i n d i c a t e s a c h a n g e in s e d i m e n t nature that we believe results from textural c h a n g e s ( T a b l e 2). B e l o w - 1 4 cm, X-radiograph d e n s i t y is s i g n i f i c a n t l y d i f f e r e n t t h a n t h e t o p two z o n e s i n d i c a t i n g a c h a n g e in s e d i m e n t d e n s i t y a n d t e x t u r e . Diffiase r e m n a n t s o f h o r i z o n t a l p r i m a r y structure and limited bioturbation characterize the l o w e r z o n e to c o r e b o t t o m a t ~ 4 0 c m . C o r e 8 c o n t a i n e d two d i s t i n c t s e d i m e n t a c c u m u l a t i o n r a t e s t h a t c o r r e s p o n d , w i t h i n t h e r e s o l u t i o n o f o u r samp l i n g , to t h e two u p p e r m o s t X - r a d i o g r a p h i c z o n e s (Fig. 2C). T h e m o s t r a p i d a c c u m u l a t i o n (0.199 c m yr -1) was m e a s u r e d w i t h i n 0 - 9 cm. T h i s r a t e r e p -

resents a maximum value based on the evidence o f l i m i t e d b i o t u r b a t i o n in this z o n e , w h i c h w o u l d i n c r e a s e a p p a r e n t a c c u m u l a t i o n rates. P r i o r to this time, a much slower rate of accumulation occurred (0.047 c m yr -t, 9 c m to - - 1 4 c m ) . F o r this c o r e , r e s o l u t i o n is a p p r o x i m a t e l y 5 - 1 0 yr f o r t h e u p p e r z o n e a n d 2 0 - 5 0 yr f o r t h e s e c o n d z o n e . E s t i m a t e d d a t e s f o r t h e 9 c m a n d 14 c m h o r i z o n s a r e t h e m i d 1940s to m i d 1950s a n d m i d 1800s respectively, with s e d i m e n t d e p o s i t i o n b e l o w 14 c m q u a n t i f i e d o n l y as p r i o r to m i d 1800s. Discussion

HYr'oxtc AREA: CORE 10 C o r e 10 (Fig. 1) is t h e c l o s e s t to a d i s t r i b u t a r y m o u t h ( - - 3 3 k m , 29 m w a t e r d e p t h ) , a n d in a n area of documented persistent seasonal hypoxia ( R a b a l a i s et al. 1991). A s e d i m e n t a c c u m u l a t i o n rate o f 0.55 c m yr -I (i.e., 1 c m ~ 2 yr) at this c o r e site a l l o w e d analysis b a c k to a b o u t t h e t u r n o f t h e

878

T . A . Nelsen et al.

Quartz: % of Coarse

A

5O

60

70

80

913

Benthic Forams: % of Coarse

B

0

1990 "1-

1990

197C

1970 -i-

4

8

16

12

-.-.v.--::..il i t

9 m:::"

Forams

~ O~176176

~o

....

9

9 o

=

,

,

"'~...',,,, ,',,

1950

'

Quartz

"V""

1950 T

...-:::::'5 "::""::i

Glauconite ,

II'

:

- - - # : ~ -

~

m

1930

22::,,t '" .... ;i""

o I

1910

9 9

eo

Coarse %

1910

="

"" .....

w"

i

/

0.0

5.0

"""" ::. . . . . . . . .... . . . . . . . .

[

2.00

4.00

6.00

8.00

Sediment Coarse %

10.0

15.0

20.0

25.0

Glauconite: % of Coarse

Fig. 3. Distributions and trends of sediment coarse fraction (:>63 f~m) components in core 10 showing (A) total coarse fiaction (solid) and quartz (dashed), and (B) glauconite (solid) and benthic toraminifera (dashed).

century. Thus, it was ideal for observing changes, if any, in sediment c o m p o n e n t s due to a n t h r o p o genic influences d u r i n g the last --90 yr. Detailed (1-cm intervals) analysis of the sediment's coarse fi'action (>-63 I~m) indicated a complex and variable record of q u a r ~ within the core (Fig. 3A). A linear trend analysis of these data indicates an u p c o r e decline ( - 8 2 % --> 67%) in lithogenic quartz, a b u n d a n c e that is consistent with declining trends o f sand transport for the Mississippi River (Meade and Parker 1984). In contrast, the total coarse fraction displays an increasing u p c o r e trend indicating that the a b u n d a n c e changes of this portion of total sediment are controlled by factors o t h e r than lithogenic quartz (Fig. 3A). During microscopic analysis of the >-63 ~m fractions, a grain type was observed whose very high a b u n d a n c e was not only u n i q u e to core 10 but also limited to the -->63 I~m fraction. These grains exhibited coloration in various shades o f green and

ranged fi-om - 6 3 I~m to 200 ~m in diameter. Initial chemical analysis indicates a composition consistent with the solid-phase glauconite. Analyses o f core 10 samples, using SEM/EDA, indicate a major-element composition of Si, A1, Fe, K, and Mg, which corresponds well with the known major ele m e n t composition of glauconite; the general formula (Odin 1988 [p315]): of which is K(x+y) (Si4 x,Al• _4 (Fe+3A1,Mg,Fe ~2) _ 2 0 1 0 ( 0 H ) 2

In terms o f grain bulk chemical composition, glauconite grains are intermediate between typical river-borne clays and a highly evolved 10-angstrom " r e f e r e n c e " glauconite (T. H o o d personal communication). Using SEM analysis, a variety of clay morphologies from vermicular forms to wavy platelets were observed on the grain surfaces. These clays were, in some cases, growing across cracks in quartz, grains, which strongly suggests that at least a subpopulation o f these grains was f b r m e d in situ.

Sediment Trends and Anthropogenic Influences

Assignment o f these grains to the traditional glaucony or verdine facies (Odin 1988) awaits X-ray diffraction in an ongoing detailed investigation ( H o o d et al. in preparation). This ongoing chenfical and physical study of glauconite, from the shelf region adjacent to the Mississippi River, is based o n a network (38) o f grab samples uniformly covering the area o f Fig. 1 as well as at the Mississippi River m o u t h and upriver ( H o o d et al. in preparation). These samples were taken to address the geological principle " t h e present is a key to the past," with the specific intention o f providing insight into surficial sediment composition and distribution o f glauconite. Results to date, and g e r m a n e to this discussion, indicate that shelf glauconite is chemically and physically (size) distinct from the river-borne p o p u l a t i o n and e n r i c h e d in Fe, Mg, and K relative to n o r m a l shelf sediments. Moreover, the highest p e r c e n t a g e of surface glauconite was observed in areas o f docum e n t e d hypoxia (Nelsen et al. 1994), with size-distribution data indicating that the river-derived c o m p o n e n t accounts for - 2 5 cm, ---early 1940s) of core 10 resembles that f r o m n o n h y p o x i c area surface grab samples. In contrast, n e a r core top, the assemblage is similar to that observed in surficial samples f r o m hypoxic areas.

To e x a m i n e quantitatively the e x t e n t to which the benthic f o r a m i n i f e r a p o p u l a t i o n may have c h a n g e d over time, the S h a n n o n - W e i n e r Diversity I n d e x (SWDI) was calculated for each of the seven levels e x a m i n e d in core 10. T h e results indicate that the diversity decreases f r o m n e a r - 1 . 0 at core base (--1900) to - 0 . 7 0 at the core top (--1991). T h e greatest c h a n g e o c c u r r e d between 1930 a n d 1960 (Fig. 4B). It has b e e n established in previous studies that variation in p o p u l a t i o n diversity can be used to m e a s u r e the d e g r e e of p e r t u r b a t i o n by m a n (Patrick 1983). T h e r e f o r e , we believe that the water-quality variability (i.e., hypoxia) plays a role in o u r observations. T h e t e r m hypoxia indicates a d e g r e e of oxygen depletion that would induce a severe stress on marine m a c r o b e n t h i c organisms. This has b e e n observed in o u r study area (Rabalais a n d H a r p e r 1992). t I y p o x i a is operationally defined, in the n o r t h e r Gulf of Mexico, as 2,500 grains g-t of sediment. In contrast, core 7 glauconite was below the detection limit (Table 2). For benthic foraminifera, the S~q-)I o f core 7 indicates h i g h e r diversity (mean = - 1 . 2 1 ) t h r o u g h o u t the corc (Fig. 5). In contrast, benthic foraminifera in the hypoxic-arca core und e r w e n t an u p c o r e reduction in diversity from the early 1940s to the early 1960s (Fig, 4B, means = - 1 . 0 6 versus - 0 . 6 8 ) . We believe that these data provide a c o g e n t a r g u m e n t for linkage of benthic foraminifera species shifts with the onset and growth o f hypoxia at the core 10 site. Moreover, the c o n c o m i t a n t post-1940s increase in glauconite a b u n d a n c e , along with United States fertilizer usage, provide additional evidence for linkage between these diverse parameters. Organic carbon data for core 7 provide some valuable insights into shelfwide OC accumulation and are c o m p a r a b l e to values observed in core 10

882

T. A. Nelsen et al.

Diversity Index -1.50

-1.00

-0.50

0.00

MISSISSIPPI CANYON: CORE, 8

i

D

OC I

0.80

1.00

1.20

(-->1985, Rabalais et al. 1991, 1992), these data suggest that elevated O C % may be a c o n t r i b u t i n g to, but not a d o m i n a n t factor in, hypoxia a n d glauconite formation.

1.40

% Organic Carbon

Fig. 5. T h e S h a n n o n - W i e n e r Diversity I n d e x for benthic foraminifera (bars) a n d the organic carbon % (squares), over the estimated time span o f ~ 1900-1980, tor core 7, an area of n o n hypoxic conditions.

(Eadie et al. 1994). C o r e 7 data show a general u p c o r e increase in O C % (Fig. 5) f r o m 0.84% at core base (~1900) to 1.21% n e a r the core top (mid 1970s), which closely parallels values and rates of c h a n g e r e p o r t e d for core 10 (0.75% 1930 increasing to 1.45% in the surface layer; Eadie et al. 1994). T h e c o m p a r a b l e trends o f these two cores suggest that organic c a r b o n p r o d u c t i o n and burial are fairly u n i f o r m across the shelf a n d have b e e n for the time periods r e p r e s e n t e d by these two cores. Moreover, with the absence o f glauconite, a h i g h e r m e a n SWDI, a n d n o docum e n t e d evidence o f r e c e n t hypoxia at this site

Core 8 was recovered f r o m the Mississippi Canyon in 933 m o f water a n d - 7 6 km f r o m Southwest Pass (Fig. 1). S e d i m e n t a c c u m u l a t i o n at this site was significantly lower than at previous sites and is characterized by a near-surface (-